In the past few months, I have noticed several hundred files, mainly TIFFs, showing a 'down arrow' flag for unsaved metadata. Every combination of reading/writing metadata to these files has been giving me either a quick return of the down arrow or an actual message "Unable to Write Metadata to files". I've read reports that this error has several possible causes.
Today I found a new 'fix' that worked on all such files for me: enabling 'Automatically write changes into XMP' under Catalog Settings.. > Metadata
Historically I had this set, but late in troubleshooting my problem, I discovered it unchecked.
These were TIFF files from Epson flatbed scans of prints made more than 10 years ago with SilverScan. I tried reading/writing the metadata to the files many times and in various ways, all in vain. Before I discovered my 'fix', I had resorted to stripping the XMP data from some of the files with exiftool, which 'fixed' the metadata-save problem (on some files) only to destroy important information including any Develop work on the files.
With some of these 'XMP stripped' TIFFS, now showing no immediate 'down arrow' warnings, converting them to DNGs and then trying to edit their metadata or Develop settings brought the 'down arrow' back again -- even as DNGs, they were again 'unsavable'.
Strangely I could see that in fact the file's metadata was changing and was being updated, as shown by the full textual metadata output from exiftool--but still, the down-arrow flag continued to show.
As soon as I turned on 'auto write XMP', all the 'down arrows' on a few hundred files across several Library folders quickly disappeared.
I do not understand how or why this fixed the problem. Presumably, it depends on some difference between a manual 'Save Metadata to File' from the Menu commands compared with auto-writing of metadata into XMP.
MacOS Ventura 13.3.1, Lightroom Classic 12.3, Mac Mini 2018 i7 with 32GB RAM, catalog stored on the Mac, image files on a TrueNAS SMB server accessed over local Ethernet network.