I haven’t seen this issue yet.
I have a number of buttons on my app screen, all default square. The last two buttons I’ve added are rounded at the corners as soon as they were dragged onto the screen, before I size them or give them a color, and I can’t see how to make them square like the others. They look like I have put “25” into border radius or border radii, but both are blank. And if I put 25 or 50 into border radius, nothing happens, and then when I clear that, the buttons stay round. Finally, if I duplicate a square button, it’s rounded as well.