1. XMP files are not being written automaticallty. I've checked the appropriate catalog settings (Automatically write...); I've checked to see that the background "saving xmp" process is running, even toggling it off/on. As far as I can see, the XMP files never get written by any kind of automated background process.
2. But there is something even stranger going on. If I go to an individual image that I have edited, and use the metadata ==> save metadata to a file, occasionally, an XMP file is written that has the edits listed -- I can read them by editing the XMP file in textedit. (Those files are typically 10K or thereabouts, but can be larger.) But then, if I do exactly the the same thing with another edited file, the XMP file will be a 4K file, with only basic camera metadata and none of the editing/change information. And those "bare" files are always the same. On the rare occasion the XMP file does get written correctly by this method, I have no idea why.
3. My image files are on an external hard drive; the catalog is on the internal system drive. Macos 12.6. and here's the real kicker. I bought a Macbook 16" laptop, and copied the catalog and files to that computer. And the same problem happens on the laptop. -- XMP files not written automatically, and usually, when creating them manually, they contain only limited data, not the edits.
If it helps at all in this discussion, I have 96,000 images. Since 2019, SONY A73R raw images.
It's driving me crazy. Could this be a catalog issue?