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A few additions.

• This issue was already fixed in V23-2-0. Just updating your idf file solves your problem.
• This issue is not fixed yet in V23-2-0. At this moment, the hot water flow rate supplied to SwimmingPool:Indoor varies according to the heating demand, and the excess water is bypassed.
• Design Loop Exit Temperature of 75°C in Sizing:Plant is too hot. It should be 35°C to 45°C, but no worries, the supply water temperature is 35°C because your supply water setpoint (Sciorba Temperatura AC Vasca) is 35°C. Sizing:Plant is only used to autosize the boiler capacity[kW] (= Specific heat of water[kJ/(kg·°C)] * Density of water[kg/m3] *Pool Heating System Maximum Water Flow Rate[m3/s] * Loop Design Temperature Difference[°C]).
• Minimum Loop Temperature in PlantLoop should be 15°C (Make-up Water Temperature) rather than 27°C (Pool Setpoint Temperature).
• Make-up Water Supply Schedule at DesignDays (i.e., "Hot Water Temperature Summer Design Day 3" and "Hot Water Temperature Winter Design Day 3") is set to 60°C. I think this is a mistake. It should be 15°C.
• I think there is no way to set the initial condition of Indoor Pool Water Temperature. EnergyPlus uses a warm-up period to achieve a periodic initial condition for simulation. After the warming up period, EnergyPlus sets the first day’s starting conditions the same as the end of that day. You can post a new issue as a NewFeatureRequest to request the additiona input field for the initial condition of the pool water temperature.
• Unlike reality, EnergyPlus does not consider Average Depth to calculate heat transfer between a swimming pool and adjacent environments. It is only used for calculating the water volume.

But what is more interesting to me is that your earlier model suggests another bug in SwimmingPool:Indoor: When the heating capacity of the heat source (e.g., boiler) is insufficient, it can happen that the hot water loop (boiler and pump) does not turn on even if the pool water temperature is below the setpoint.

I reproduced the bug with the ExampleFile 5ZoneSwimmingPool.idf. I revised the model a bit:

• Nominal Capacity of Boiler:HotWater was hardsized to 10000W.
• The heating setpoints for zones and outdoor air were set to -50°C so as to eliminate space heating.
• Setpoint Temperature Schedule of SwimmingPool:Indoor was changed from always 27°C to the following.

(This change is important. EnergyPlus developers should test not only at the constant setpont but also at the variable setpoint).

Schedule:Compact,
    PoolSetpointTempSched,   !- Name
    Any Number,              !- Schedule Type Limits Name
    Through: 12/31,          !- Field 1
    For: AllDays,            !- Field 2
    Until: 6:00,  16.67,     !- Field 4
    Until: 20:00,  27,       !- Field 6
    Until: 24:00,  16.67;    !- Field 8

The chart below shows the hourly results of Indoor Pool Water Temperature and Indoor Pool Inlet Water Mass Flow Rate during the first two weeks of January. Boiler:HotWater and Pump:VariableSpeed never turn on after the first week.

Files are here.

A few additions.

• This issue was already fixed in V23-2-0. Just updating your idf file solves your problem.
• This issue is not fixed yet in V23-2-0. At this moment, the hot water flow rate supplied to SwimmingPool:Indoor varies according to the heating demand, and the excess water is bypassed.
• Design Loop Exit Temperature of 75°C in Sizing:Plant is too hot. It should be 35°C to 45°C, but no worries, the supply water temperature is 35°C because your supply water setpoint (Sciorba Temperatura AC Vasca) is 35°C. Sizing:Plant is only used to autosize the boiler capacity[kW] (= Specific heat of water[kJ/(kg·°C)] * Density of water[kg/m3] *Pool Heating System Maximum Water Flow Rate[m3/s] * Loop Design Temperature Difference[°C]).
• Minimum Loop Temperature in PlantLoop should be 15°C (Make-up Water Temperature) rather than 27°C (Pool Setpoint Temperature).
• Make-up Water Supply Schedule at DesignDays (i.e., "Hot Water Temperature Summer Design Day 3" and "Hot Water Temperature Winter Design Day 3") is set to 60°C. I think this is a mistake. It should be 15°C.
• I think there is no way to set the initial condition of Indoor Pool Water Temperature. EnergyPlus uses a warm-up period to achieve a periodic initial condition for simulation. After the warming up period, EnergyPlus sets the first day’s starting conditions the same as the end of that day. You can post a new issue as a NewFeatureRequest to request the additiona input field for the initial condition of the pool water temperature.
• Unlike reality, EnergyPlus does not consider Average Depth to calculate heat transfer between a swimming pool and adjacent environments. It is only used for calculating the water volume.

But what is more interesting to me is that your earlier model suggests another bug in SwimmingPool:Indoor: When the heating capacity of the heat source (e.g., boiler) is insufficient, it can happen that the hot water loop (boiler and pump) does not turn on even if the pool water temperature is below the setpoint.

I reproduced the bug with the ExampleFile 5ZoneSwimmingPool.idf. 5ZoneSwimmingPool.idf (V23-2-0). I revised the model a bit:

• Nominal Capacity of Boiler:HotWater was hardsized to 10000W.
• The heating setpoints for zones and outdoor air were set to -50°C so as to eliminate space heating.
• Setpoint Temperature Schedule of SwimmingPool:Indoor was changed from always 27°C to the following.

(This change is important. EnergyPlus developers should test not only at the constant setpont but also at the variable setpoint).

Schedule:Compact,
    PoolSetpointTempSched,   !- Name
    Any Number,              !- Schedule Type Limits Name
    Through: 12/31,          !- Field 1
    For: AllDays,            !- Field 2
    Until: 6:00,  16.67,     !- Field 4
    Until: 20:00,  27,       !- Field 6
    Until: 24:00,  16.67;    !- Field 8

The chart below shows the hourly results of Indoor Pool Water Temperature and Indoor Pool Inlet Water Mass Flow Rate during the first two weeks of January. Boiler:HotWater and Pump:VariableSpeed never turn on after the first week.

Files are here.