Garage Floor Sinking After Coating Foam Leveling?

A friend of mine recently had their garage floor coated with polyaspartic. The floor was already in rough shape before cracks, stains, and some chips but the coating made it look great. Since then, they’ve noticed that parts of the floor have sunk in a few spots.

They’re not entirely sure if some of the sinking was there before the coating.

They’ve read that expanding foam injections can sometimes level concrete slabs and are wondering if this could work on a floor that’s already coated.

They’ve also heard that cutting relief joints might make it possible, but they’d like to hear from anyone in the business whether this approach is actually feasible.