I was taught as a young man a secret trick to know if your used car you were seeking to buy had been taken care of well, or not. Run your fingers along the inner edges of the wheel housings. If there is an inch thick solid cake of mud they never ever washed out, you’ll suspect the rest of the car maintenance history might be spotty as well. Only people who truly love their cars wash out carefully the confines of the fenders.
Yes that caused many old fenders to rot out.
Not related though to the problem here. Problem here is a drainage flaw in the design of the accessory mud guards. The ten pounds of dirt that collected inside - it’s not washable in the little space provided because the drainage hole is to small- was causing the piece of bumper cover to flex outward. Each one was filled to the top.
Just pulled mine for trailer hitch install, and found one side full w/ 10 lbs of mud and the other side basically empty. Looking at the design that lip at the bottom has about an 1/8" hole where the fender liner and the guard overlap, and it's basically a funnel for anything running down the rear of the liner. Guess it was cheaper to incorporate that drain hole than it was to redirect the the drain water path.