This is a recent, intermittent issue (I've used LR for years, and only seen this recently). I'm running Lightroom Classic CC release 7.5. When merging photos to HDR, Lightroom will process the images and at the moment it creates the DNG it sometimes outputs an error "Some save metadata operations were not performed". It then states "Photos have read only access (1)", and lists the new dng file that was just created. This also happens occasionally when merging to Panorama.
To try to debug this, I made sure that the folder containing all my raw files had no read-only files in it (I did this in the properties pop-up for the folder, clearing the "read-only" attribute box and applying that setting to all files in the folder). This was done BEFORE any HDR merges were performed. If there's a read-only file involved, its something LR is creating temporarily during the merge.
I also checked the files in question after the error (both the raw images and the new dng): none of them are read-only.
I can also state that this doesn't happen consistently. I had 30 sets of bracketed images that I set to processing all at once in the background (using ctrl-shift-H on each set of 3). The error popped up for only 2 of the 30 sets.
One other unfortunate side-effect of the problem is that it halts all background tasks until its acknowledged. That's the big reason I'm logging it and not just ignoring it: I routinely turn a large queue of background tasks loose, then leave and enjoy life elsewhere. Coming back several hours later to find out nothing has happened is a drag.