We're on Forms 11 Update 4, and this issue seems limited to the Modern Designer.
We've noticed that the writable portion of the "Draw" tab in the signature box doesn't fill the entire space. If you try to start your signature all the way to the left, or anywhere outside of where I've colored in black in the attached image, nothing writes into the box. We thought it wasn't working at all until we realized starting further to the right allowed us to draw a signature.
I assume this is a bug, but if there's a solution, please let me know.