Photos app - auto merge incorrect people

Situation: the photos app has automatically merged people which should not be merged and I cannot undo it.

Background: My iCloud photo library has +100,000 items, I've digitised my entire family photo album and invested countless hours over the past few years adding meta data (like locations and backdating), building a file structure and ensuring every photo with a face is tagged using the 'people' function of the photo app. I predominately do this on my Mac.

I discovered today that the software has automatically merged several people (impacting several hundred photos), in some instances both merged people are in the same photos and in other instances they are not (some people are not related, the same gender or look alike!). Once over the immediate frustration, I began manually re-tagging the mis-merged faces only for the software to 'undo' my tags and re-merge the same people a few minutes later.

I spoke with Apple's creative services support team today who were able to see the issue remotely. We played around with tagging to confirm the auto mis-merge. They've asked me to leave the app running for 12 hours for the library to update. I don't really know how that's going to begin resolving the issue. The only real change has been the purchase of a new iPhone which has begun doing it's own scan of my photo library - that's the point things started to go wrong.

Does anyone else relate to this? Has anyone found a fix?