My guess is that the AABB is the issue. Likely Godot looses a corner of the tilemap in the camera view margin, which causes the image not to be drawn (though admittedly I am guessing that Godot uses something like AABB for 2D visibility).
I would see if cutting the sprite down into multiple smaller sprites fixes the issue. It should also help performance a bit, as there will potentially be less overdraw since smaller sprite can be drawn rather than always drawing the big sprite and showing just a small section of it.
