[B3.1][Stadium] Signs go black after shadow calculation

[YonCan]

I built a track in the stadium editor with custom signs and original nadeo signs. After validating I continued with shadow calculation. After that I wanted to drive the track again and clicked the (green) flag. On this turn all nadeo signs were black (not the I-didn't-find-the-sign-file black and red pattern, only black), the custom signs remained as they should.

After exiting the editor and loading the map in local solo mode, the nadeo signs were back - as well when returning to the editor.

I tested this with fast and standard shadows.