Nobody has any insights?
Me, not much.
C1 never writes to existing image files. You can only make C1 copy metadata from its internal Metadata storage (in Session or Catalog) to XMP sidecars. "Sync" in C1 lingo. PM can pick up C1 metadata written to XMP sidecars , sync'ed, by C1. Provided C1 writes to a XMP element that PM reads. Sounds obvious, but there is a thing with image file type on the PM side of the fence. PM does not bother with XMP sidecars for some image file types. I don't know about TIFFs, but I had to give up on transferring metadata from C1 to PM for DNGs. PM reads DNG metadata from the DNG file only, PM ignores DNG XMP sidecars.
I have not played with this for a wile, but I think C1 is liberal, kind of, when it comes to picking up metadata embedded in image files and/or in XMP sidecars. "Load" or "Reload" in C1 lingo. Try to set the C1 preference Auto Sync sidecar XMP to None and experiment with one or a few image files (raw, tiff, jpeg,dng...) using the C1 Metadata tool. Change some value, do a Sync Metadata manually (Metadata tool "..."-menu). Check if PM pics up the changed metadata. You may want to try several fields, C1's displayed field names do not necessarily match those of PM (nor IPTC guidelines for that matter).
Color tags are special as exchange requires exact same spelling in both programs. "Green" and "Keeper" are different even if both program happens to display a Yellow color patch :-) In your case that is probably taken care of if it works with some file types.
C1 suffers from a kind of race condition when it comes to Metadata and the "Full Sync" that many C1 users apparently prefer. Speed penalty is one reason for not using it, said race condition is another. The C1 forum has a high number of postings where Full Sync users wonder why C1 overwrites Metadata they just entered in the Metadata tool. Ratings, color tags, keywords..., you name it. C1 Stores the new metadata internally, then reads from XMP/image file and restores old values. C1 still does not have separate settings for Automatic Load Metadata and Automatic Write Metadata, you only get Automatic Load or Automatic Full Sync (load and sync). The manual Load/Sync in the Metadata provides separate controls.
With that background: You have C1 set to prefer metadata in XMP sidecars over metadata embedded in the image file. Your PM setting "Always create and/or update sidecar files" is Off (not ticked).
