For those interested in the resolution of this issue… the Extinction DLC and devkit added 3 new Named Target Definitions in PrimalGameData, Robots_Neuitral, Robots_Aggressive and Corrupted.
My mod PrimalGameData didn’t receive these… and because of this (and probably a bunch of blueprint scripting related dependencies), Defense Units weren’t able to damage wild dinos and were generally behaving wrong. Somehow Enforcers and Corrupted still functioned ok (at least they seemed to).
Anyway, once I added those 3 missing definitions into my PrimalGameData blueprint… presto, the issues all went away.
Hope this helps someone.