Grey shapes/artifacting?

Ignore the rushed drawing it's a WIP and I am also not good at drawing just yet.

I was drawing on my tablet and decided to save my project and take a screenshot to share with friends, once I shared it I saw square-ish shapes appear in the pic but not on the tablet, switching my HDMI cable to another monitor and goin to kirita the square are now visible, why?

Also tried using the fill/bucket tool one one of the shapes but everything on that layer dissapears so I have to erase them manually but really want To know why that happened thx.