I suspect the reason functionCall doesn’t work is related to the fact that Desmos treats function products very inconistently. I made a note about that in the code for Function Transformations in af(k(x-c))+d Form | Component Based Approach. If that is indeed the reason, it will require Desmos to rewrite some handling on its end to change that.
Bad news is that code is complicated. Good news is that it works every time.
If there’s anything I can do to make it simpler to implement, please do ask. I could take a look at it again.