XMP issue, Lightroom Classic 11.5, MacOS 12.6 (iMac 5K, 27", 2017) (Macbook Pro, 16" 2019)

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?