I think I found the problem.
The back side has a different color (Background Color) which is black by default.
The problem is that Background Color has a bug (I think).
It has a very strange behavior.
Opacity is not working and colors are not updating correctly.
Also all changes are lost on compilation… and the text turns black even if the Background Color property is not black.
Is it a bug?
(I’m using UE5.4.4)