Global Layout disappears when returning to a layout

I have a game with 'Rooms'. Every level has the same layout, layer0 for the room and UI for the HUD.

The HUD draws fine most of the time.

However, as an example if I go as follows, the HUD doesn't display...

Room 1 > Room 2 (OK)

Room 2 > Room 1 (OK)

Room 1 > Room 2 (OK)

Room 2 > Room 1 (Disappears, never to be seen again.)

If I wander room to room it generally works, until I start reloading some old rooms.

Any thoughts?