I am in the same boat as you jonwright:
Mediabay always keeps scanning VST3 presets, in my user folder.
The very weird thing is that all the VST3 presets once were scanned fully complete, cause all the VST3 preset folders from all my plugins were colored WHITE then in Mediabay, meaning those folders were ready scanned 100%.
But next time i open Cubase and i open Mediabay (be it as seperate window or in Right Zone), i can’t select any media cause the scan process kicks in again and takes forever (err, well let’s say about 5 minutes), to have scanned all the VST3 plugin presets folders again.
I disabled the VST3 presets folder many times in Mediabay, but next time i start Cubase, it’s again selected and so it isnt anymore excluded from scan.
So i just can’t use Mediabay at this point, so frustrating…unworkable …