Why does running EnergyPlus with an old IDD no longer fail?

edit

I'm updating eppy to be able to use EnergyPlus v8.9.0 (I know, I know, v9.0.0 is just coming out, we'll get there), and one test is failing unexpectedly. This test asserts that if EnergyPlus is called with an IDD version that doesn't match the IDF version, the run fails.

I've tested this outside of eppy and it seems to be an issue with EnergyPlus itself. Is this expected behaviour?

Call:

C:\EnergyPlusV8-9-0>energyplus.exe --idd "C:\Users\<snip>\eppy\resources\iddfiles\Energy+V7_2_0.idd" "C:\Users\<snip>\eppy\resources\idffiles\V8_9\smallfile.idf"

Std out:

EnergyPlus Starting
EnergyPlus, Version 8.9.0-40101eaafd, YMD=2018.10.02 11:35
Adjusting Air System Sizing
Adjusting Standard 62.1 Ventilation Sizing
Initializing Simulation
Reporting Surfaces
Beginning Primary Simulation
Initializing New Environment Parameters
Warming up {1}
Starting Simulation at 12/21 for DENVER_STAPLETON ANN HTG 99.6% CONDNS DB
Initializing New Environment Parameters
Warming up {1}
Starting Simulation at 07/21 for DENVER_STAPLETON ANN CLG .4% CONDNS DB=>MWB
Writing tabular output file results using HTML format.
EnergyPlus Completed Successfully.

eplusout.err

Program Version,EnergyPlus, Version 8.9.0-40101eaafd, YMD=2018.10.02 11:35,
       ************* Testing Individual Branch Integrity
       ************* All Branches passed integrity testing
       ************* Testing Individual Supply Air Path Integrity
       ************* All Supply Air Paths passed integrity testing
       ************* Testing Individual Return Air Path Integrity
       ************* All Return Air Paths passed integrity testing
       ************* No node connection errors were found.
       ************* Beginning Simulation
       ************* Simulation Error Summary *************
       ************* EnergyPlus Warmup Error Summary. During Warmup: 0 Warning; 0 Severe Errors.
       ************* EnergyPlus Sizing Error Summary. During Sizing: 0 Warning; 0 Severe Errors.
       ************* EnergyPlus Completed Successfully-- 0 Warning; 0 Severe Errors; Elapsed Time=00hr 00min  0.30sec`