Factory settings for both 3.6 and 4.0 latest as of 25 June. No change. See screen recordings.
This is a bug! I've never come across this before and as you can see from the screen recording some parts of the mesh cannot be selected in wireframe view.
Retested with latest build and seems to be fixed. I will keep a lookout for it happening again.
Try moving the mesh up keeping the origin at the centre. The further up it is the further the UV island is offset from the UV space.
But if I delete the new face and unwrap again it reverts back to it original behaviour.
This file has 2 objects which were created by extruding single verts and adding a face. Shape 1 has a UV island that forms outside of the UV space and shape 2 doesn't create an island at all. The…