Can you try the reverse.

Create an empty Catalog and from there import from the current working catalog. Use the option which leaves the images in their current location.
Do a check for Missing images first, and resolve all missing images to your satisfaction.

I use a Samsung T9 as the permanent home for my catalog, last 2 years images and my office docs. I use it on Windows 95% of the time and MacAir for travel, with no issues. I do have to update the current location for Cloud Downloads when I switch to Mac.. which I can ignore as I do not use LrCloud apps to import to LrCloud.

Can you try the reverse.

Create an empty Catalog and from there import from the current working catalog. Use the option which leaves the images in their current location.
Do a check for Missing images first, and resolve all missing images to your satisfaction.

I use a Samsung T9 as the permanent home for my catalog, last 2 years images and my office docs. I use it on Windows 95% of the time and MacAir for travel, with no issues. I do have to update the current location for Cloud Downloads when I switch to Mac.. which I can ignore as I do not use LrCloud apps to import to LrCloud.

Thanks, Matt. That's an interesting diagnostic test. I already have a reliable workaround (export to the internal APFS SSD, then copy the exported folder to the external SSD with Finder), and I'm leaving for a trip shortly to photograph the August 12 total solar eclipse in Spain. I'll try your suggested import-into-a-new-catalog workflow after I return and report whether it behaves differently from Export as Catalog.

Vg.. I was going to suggest your workaround also. As you will be changing the catalog Id linked to LrCloud you may need to relink the new catalog to the cloud. That might be a small or big job. The process is documented on these boards.

Good luck and clear skies in Spain..