Update:
it looks like it’s an actor ID problem.
The glass that doesn’t flicker is a Static Mesh Actor with ID 100 or similar.
The flickering glass is a blueprint I added much later, with ID 4000.
In fact, if I delete the ID 100 actor and place it again in the level, It gets ID >4000 and starts flickering.
Maybe actors with high ID get “rendered” later and if they have translucent materials they show this kind of problem?
Can I edit the blueprint actor ID to be lower?