2D Animation Fill tool is ignoring Grease Pencil lines?

I am really struggling to figure this out, any help would be really appreciated! I am drawing a character in grease pencil and have different parts of the arm on different layers. I am trying to add color to the drawing using a new layer with the fill tool, but when I click on the canvas the fill tool acts as if there are no lines at all and fills the whole visible screen. I have all layers locked except the layer I wish to add the fill on, and the tool is set to ‘Visible’ under Advanced->Layers->Visible.

I am fairly certain there are no gaps between the lines (except they are on different layers) that the fill tool could ‘bleed’ through, just in case I tried adding ‘construction’ lines and Shift-Clicking and outlining with the fill tool, but I still get the same problem.

What’s even stranger is if I zoom ‘out’ on the screen the fill tool will sometimes work normally, only filling inside the lines where I clicked. But this almost seems to work at random and when I zoom back in to a normal size it breaks again.

This is driving me absolutely crazy and no tutorial I have watched has helped me figure out what I’m doing wrong! I took a screen recording of the problem as I’m not really sure how to describe it: