Yeah that happens to almost all YJs no matter the weight of the driver. I suppose the heavier the driver the worse it gets but it happens to most all of them. Somehow I’ve been lucky and it hasn’t happened to mine.
I think part of it is due to the stamped steel seat brackets, they have a part that sticks out right there and presses hard into the floor at the bolt hole locations, and then the weight of the passenger or driver puts lots of pressure on those points.
Get them welded up and it should be fine.