There was a glitch a while ago where if you paint bucketed between 2 pixels it would show one, but then way later would swap to the other pixel and mess up the drawing. It hasn't happened to me in a long time so I assumed it got patched.

You MAY be able to fix it by changing the drawings aspect ratio, however you might not be able to, since you can't do it in app.

What device are you using. If you are using Samsung then i know how to change it.

But that might not even work.

Whenever it happened to me I just drew over the destroyed parts. It takes a long time but it's better than restarting.