I've been testing digiKam importing face regions from Lightroom face region detection.
At first it imports the Lightroom face regions okay, but if a new face region is added into this picture using digiKam, this new face region isn't going to be read in Lightroom. Going further, if I get this very picture and now add a new face region in Lightroom, this new face region isn't going to be imported into digiKam anymore.

Answering but not solving my previous post.

I've tested digiKam 7.2 beta. It improves a lot in interoperability of face region data, but doesn't solve the face region problem entirely:
- digiKam will read Lightroom face regions if you search for new items.
- if a photo is exposed to Lightroom, both softare will read detected face regions.