I think his ex just made it up so no one would find out he had a relationship with another man
No, as a partner of someone I’d be concerned and confused if they disappeared after I did so many things for them, He’s not a stalker he was just concerned where his partner went and was upset that his partner didn’t say anything.
He came to that room without their permission and his ex is totally mad at him, shouting at him so they thought him as a stalker, and he's not a familiar face, i guess.
In chapter 10 we also see a piece of this memory when the seed remembers it. The ex said in front of his friends "get out of here, dirty homo". And we see the seme cry and we also notice that the ex already has the mark on the cheek.
I hope I don't sound dumb but how is he considered a stalker?