RESOLVED, extremely noob loophole error here.
The Master Mat. I made has a wrong AO placeholder when a Subtance AO texture is not used/loaded in the relative Mat. Instance texture parameter:
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This lead to the Material Instance of the walls around the door to be pitch black (AO=0); I had movable lighting so the Walls material looked fine in the viewport but still affecting the reflection on the metal plate!
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I changed all the direct/indirect light to static, I baked it and actually fixed the problem of the metal plate, but the walls instantly showed up their AO=0 becoming black
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Now I just have to find a correct AO texture placeholder and remove that embarassing _AO switch.
At least I learnt something and I didn’t rage quit.
Thanks endlessly and sorry for the false alarm MostHost LA
