I agree, this is happening with me. I'm curios if the problem will go away once I get a Mac with Apple silicone. I'm currently running a Mac Pro 6,1 (trash can) and an iPhone XS and they seem to fight each other. For example, there is a photo of one of my daughters that I set as a key photo for her on the Mac or on the iPhone and then a few hours later I come back and it's no longer the key photo, in fact, my Mac doesn't even have a name for the face in the photo any longer. I also find that when I go to my Mac and confirm additional faces I'll sometimes see a photo pop up that I know I have already confirmed in the past.
One theory I have is that ALL of the photos are on my Mac whereas I optimize storage on my iPhone which makes me think that the dataset available to both devices isn't the same (even if the scanning algorithm is the same).