Face recognition in general just isn't as good as Google Photos.
It's still an amazing piece of software and I'd never go back, but it isn't perfect yet.
Almost all my Google Photos "people" are mix-and-matched similar looking faces, so it's borderline useless. Immich isn't perfect, but it gives me the control to rerun face recognition and reassign faces when I want, even on my ancient GTX 1060.
Google photos isn't perfect either but I never saw these kind of issues when I was still using it.
Annoyingly you can't create a person that way yet with immich, but that's where digikam helps.