Emergency : Bug or not bug ?

Emergency please :

I’m about to deliver my very first real project, the billard tables.
Quite all is working well and the customer is happy with the render / technology.

Just i have a problem with some objects that randomly turn black (understand not-rendered at all).
When i got this some weeks ago, i was told to “open” the diffuse map slot putting a map into it before export, so i did that, and at the beguinning of my puzzle i set back to default white material.

Please see the joined video to see what happends :
Basically – and it is repeatable – i choose a color/material for the “Cadre” (=frame), then i change the “ambiance” (= change to fixed camera, change hdri envir and show/hide background plane with picture). When i come back to the white “studio” ambiance (so back to original hdri, background and free to move camera), frame is blacked out.
Without changing anything, i go to another ambiance, then back to studio, and the texture is ok.
So not a problem of missing file, not a problem of closed diffuse slot, as it sometimes works well.

I hope you guys understand (see video), and i hope i’m the faulty part of the “bug” and i did things wrong : i can share scene/application to Soft8soft guys to check, if needed.

Thanks to help : the final touch of a 6 months learning / production is stuck to this problem resolution.