I ran some tests and confirm that HEIC embedded metadata, as written by Photo Mechanic, is not read by Capture One (21). I did some searching through CO's knowledge base and community forums and didn't see this addressed although I bet it has been somewhere.
Basic process:
- Exported an original iPhone HEIC image from Photos.app without including IPTC data in a sidecar. (Should be no different than using Airdrop or other means to transfer.)
- Opened Metadata Info dialog in PM and filled it with test data (snapshot)
- Verified using exiftool that the data had been embedded into the HEIC file
- Imported the image into CO (synchronized folder, but that should be the same as using the import dialog.)
Result: None of the added metadata showed up in CO, just as you observed.
I also exported another HEIC image with some Photos.app metadata (caption, title, keyword) exported to sidecar XMP. That was read by CO, as I expected it would.
I did the same tests with a JPG, and as always, embedded metadata was read (and synced when changes were made) by CO.
I didn't change my Metadata preferences, which are different than yours: