I am seeing the same behavior with weight painting rather than texture painting. I've modified a mesh, and then go into weight paint mode and make a couple of weight paint strokes. Instead of Undo removing one weight paint stroke at a time, it jumps me back out of weight paint mode, and sometimes also undoes whatever step I did just prior to weight painting. Undo is going more than one step back, which makes it quite difficult to use. I'm using 2.93.5 on Windows 10.