The way the majority of players seem to be using "solvers" is to arbitrarily input a few bet sizes and lock/prevent certain actions, that actually do occur in practice, so that the majority of hands in a given range end up being mixed frequency plays utilizing all sizes at some frequency. When you do things this way no matter what betting action is being checked you will generally end up perceiving yourself to be correct "according to the solver" while causing a nonnegligible EV loss compared to the full game tree that you didn't even come close to approximating.
Imagine if you were using a chess engine and you determined that if you chose any of the top 7 best candidate moves at a given node that you made the right decision...
This is pretty much what the majority of players "using solvers" are doing. I'm not saying that these mixed frequency combos aren't mixes, I'm saying that if your input sizes only included 33% and 50%, but that the actual optimal sizes vs an opponent would have include 41% that both 33% and 50% could only ever be "close enough," never actually giving you the highest EV strategy.
I wonder just how different things would be if the first mover was named PioCALCULATOR rather than PioSOLVER.