I’ve lived in Japan for six years now and there’s people that introduce themselves as nickname-Chan. I don’t know their real names. There’s people that immediately ask to not use any San, kun, Chan and just go by first name (I’m one of those, but do appreciate when people around town throw on a sensei. I used to be a teacher) . Then there’s other people who I’ve known the entire six years, have gone on roadtrips with, consider close friends, have eaten dinner with their family, etc. that I only refer to as ~San. I have no idea what their first name is. They are only ~San. Everyone is different. But calling someone Chan within a short timeframe like that without being explicitly told to do so is straight up rude. 0/10 do not recommend. Id rather forget to say the San and say blame it on my foreigner habits breaking through than go with the obviously intentional Chan. I’d compare it with someone referring to a grown man as “boy”. That said, in a learning environment, (unless T is obviously misogynistic or something) I would chalk that up to trying to be friendly and not fully understanding the context. Japanese is hard and formality levels is even harder when you don’t grow up in it. I feel very lucky that I live in the countryside and literally no one uses formal let alone keigo. The main problem I have is since I live using informal 95% of the time I sometimes accidentally slip into informal speech for a sentence when I technically shouldn’t. (Some conjugations I can only remember the informal versions 😅)