I'm making a tile scroller that runs in the background of a top down style open world game. It works well, but there is this glitch where these ‘air pockets’ develop. I don't know why this happens, I've spent what feels like hours going over the code and changing things but nothing seems to work. Does anybody know what the solution could be?

Link: https://scratch.mit.edu/projects/1257810182/

Your stamp engine seems overly complex. Something as simple as:

set x to (([] - (player x)) mod (tile size))
repeat (grid width)
stamp
set y to (([] - (player y)) mod (tile size))
repeat (grid height)
change y by (tile size)
stamp
end
change x by (tile_size)
end

should work just fine.