Lightroom Classic Sync: JPG/HEIC show “Waiting for settings” in Mobile and “Error rendering” in Cloud; re-sync hangs for affected files

I have an LrC catalog with 20,000 images - mostly dng, with a few hundred jpg and heic files. Things looked normal in LrC and there was no major global operation done, but at some point I noticed that dozens of jpg and heic files showed only a preview image along with a “waiting for settings” message in LR Mobile. On the web, those images show “Error rendering”. LrC showed everything synced with no errors. I tried suspending and resuming sync on LrC - no change. I tried tweaking a develop slider on one of those files. In some cases that fixed the problem; in others nothing happened. I picked 5  of those images and removed them from “all synced images”. They were correctly removed from the cloud. When I re-added them to a synced collection, the sync of those files hung. Other files continue to sync normally. I took a diagnostic dump of the state with 5 files hung and the dozens of other files showing “Error Rendering” / “waiting for settings”. Even if tweaking settings would reliably correctly sync the metadata, I see no way to even select the problematic images - it’s dozens to hundreds of images scattered among 20,000, and they’re not identifiable in LrC, only on cloud or mobile, so I have to identify it on cloud, look at the name, find it on LrC, then tweak the setting.