Not sure if its exactly the same issue, but wanted to report on here.
1)Started yesterday with PM+ on OSX to evaluate for my use. Starting with a library of ~80,000 images on a NAS. Starting catalog sync.
2)Catalog sync completed. Metadata and preview generation background tasks kicked off.
3)Metadata completed, preview generation still going on as I went to bed.
4)During the night NAS has disconnected. Not all previews were generated (if I disconnect the network, some previews aren't there, some are there with the yellow dot).
5)Preview generation background tasks is green.
6)When I connect the NAS back, I have no way to re-trigger preview generation (as far as I can tell) nor does it start by itself (outside of full catalog rebuild which I want to avoid for now).