This is change is totally backed up by logic and is intuitive. Before 4.16 Power Material Expression used to translate to:
pow(max(abs(X),0.000001f),Y)
in 4.16 and above Power Material expression translates to:
pow(max(X,0.0f),Y)
Reason is pretty obvious: errors when using X values near 0;
Not a bug. Intended change.