When drawing a line with fluid brush, other shapes connected occasionally fill in black

I'm on the latest version of animate, and this has been an issue for a few months now. Whenever I'm drawing with the fluid brush tool, occasionally, and seemingly at random, another part of the drawing will fill in completely black, and whenever trying to control+z it, it deletes the entire drawing (or whatever was connected to the new filled in shape). I have nothing on that should make this happen, and it seems like a glitch, but I have no idea how to fix it. I cannot erase the inside black filling, and it doesn't register it as something I filled in manually, but it stays that way until I just delete and restart the entire drawing. This also duplicates when trying to select the