Yes, this is a known feature of your power level restricting the TP of your helper units; this is why all of the beginner tips you can find on this forum strongly encourage you to level at least one girl to max as soon as you are able so that your helpers wont have their power throttled back to match.

Personally, it seems like an odd system to have; it's not like helper squads soloing low level content is imbalancing. But it is what it is.