Abyssal Empowerment seems to prevent Enforced Morale from working on Demons & Demon Prince, according to the combat attributes window.
(Based on some limited experience against enemies with mez, I'm somewhat confident that this is not just a display bug - I saw the Demon Prince get stunned while showing the EM icon in the pet window )
Things work fine when Enforced Morale is used on Demonlings.
Steps to reproduce:
1) Summon Demons/Demon Prince
2) Cast Enforced Morale on pet
3) Check Combat Attributes window, mainly Status Effect Protection & Resistance. The effects from Enforced Morale will appear.
4) Dismiss pet
5) Summon Demons/Demon Prince again
6) Use Abyssal Empowerment on pet
7) Repeat steps 2 & 3 . This time the effects from Enforced Morale will *not* appear.
You can also check the HP bar of the pets - they won't even take the minor damage caused by Enforced Morale.
My guess would be that this has something to do with the "kMeter < 0.9" condition that we see on City Of Data but I have no idea what that means.