With an acrylic pan? Absolutely, insist that it be installed correctly. Acrylic will stretch and/or crack under the stress of repeated flexing.

If the pan wasn't installed IAW the manufacturer's instruction, then yes, it's a problem.

It could create a warranty problem should you ever have to go down that route.

It can shorten the life of the pan due to the repeated flexing movement of the pan surface.

It can create leaks around the drain, which can be one of the more common points of failure when pan movement occurs due to the drain/flange losing its seal, or the drain flange itself cracking.

There is no telling if it will cause problems.

If you haven't paid him in full, that money still in your hand is your best bargaining chip.

I'd contact the manufacturer's tech department and ask if a non-mortared installation voids your warranty. Ask if it can potentially shorten the life of the pan. If it can make it more prone to failure and/or leaks. That's your next chip. I don't put much faith in a manufacturer's warranty. But you use what you can to bolster your position.

Thanks so much for some good advice. I will contact the manufacturer tomorrow. I haven't paid in full yet. I just hate this kind of stuff but guess this too shall pass.

BE CAREFUL...mongoct is spot on. Subfloor can not have ANY flex. Mud, whether it's gypsum, cement or thin set will not make the flex go away. If it is one of those redi pans you tile over, be sure to check the warranty... i think it is only 2 or 3 years..is that crazy?