Photos face matching & sync overriding manual selection
I have been using iCloud Photo Library (iPL) ever since it was announced, and long before that I had diligently tagged all (30,000+) photos on my main Mac with faces. After iPL started syncing faces across devices, in the process of having all devices (3 Macs, 2 iOS devices) get into sync, a number of faces were changed by the face recognition algorithm to wrong names, including some where photos of myself are tagged as my brother. For more than a year, I have at irregular intervals tried correcting these wrongly names faces (making sure I have all five devices open at the same time and observing that the change propagates across them all). However, after a shorter or longer interval (days to weeks), when I check back in to these photos, they will have reverted to the wrong names.
Even worse are pictures where I have manually added faces that Photos don't recognize as faces. For some photos, this is perfectly fine, but for some hundreds of pictures, these manually added faces are stripped away. In attempts at beating this process, I have in some cases added 2, 3 or even 5 manual faces and named them correctly. Slowly, over time, one, then another of these is removed, while in a few cases I have made them stick.
This is extremely frustrating, especially since I cannot fathom where the wrong face ID is stored so that it makes it back after it has been changed to the correct name across all devices. Any idea how to fix this so that my carefully named faces stay correct?