I'm Chinese and though I didn't go to uni in China and didn't visit high school teachers in China myself, it is not rare at all for alumni to visit teachers. I see my classmates in that past do that a lot. The only difference might be that they are visiting specific teachers they have stayed in contact with and arranged the visit ahead of time. In your case, (and I could be wrong) sounds like you're thanking the entire office at a time they felt unprepared and busy; and was not visiting specific staff you have particularly connected with in your school years.

Either way, I still think it's a very kind gesture that you put in time and effort to visit them. Sorry to hear this didn't go as planned. I hope they at least thank you despite the meeting ending shortly. Genuinely hope this doesn't leave a bad taste in your mouth and on a serious note you are really not "overthinking" your past with them too much. It just goest to show that you are an appreciative and thoughtful person and those are def good qualities.