I first noticed this about 7 years ago. I was going with a girl, not really that serious, but regardless. Anyway, Facebook would send me a notification for friend suggestions under People you may know. These would often include her new Facebook account and her only friend on that account. We broke up but have continued to stay in touch on Messenger, but we are not Facebook friends. Every time she gets a new guy facebook sends me them as a suggested friend or people I might know. They also send me her new Facebook account as a person I might know. Watching the patterns emerge I have to admit, has been kind of interesting. There is always one guy like me, her age and pretty much mainstream, and the other guy who is usually about 30 years younger, married and kind of shady. I wonder if Facebook does this intentionally based on chatting habits?
The habit that you two are still tied together through Messenger, which is just another Facebook portal. One or both of you are poking around the other so the algorithm thinks of you as being friends. As opposed to blocking each other.
Meta owns Facebook, Facebook Messanger, Instagram and WhatsApp. I’ve gotten friend suggestions on WhatsApp and Instagram from poking around I did on Facebook.