A similar problem also occurs on an UM2: the priming being pulled onto the printbed, which would sometimes cause problems. Originally I also used tweezers (pincette), to prevent this from happening. Until I found this "filcatch" thing in a post from another user (I don't remember his name). He 3D-printed it, but I made it from inox spring steel, so it lasts forever. This prevents the primed stuff from being dragged around.
I don't know if this method for the UM2 could be adapted to work on an UM3 too?
I don't have a fix for the priming blob of filament, but for your glass, I would contact your reseller.
Discoloration on the BB Core is completely normal; PVA does burn very easily, and a thin layer of it on the outside of the nozzle shouldn't cause any issues.
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