This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Wandering Son
The general concept of the series can be this. Transsexual children growing up.. Anyone who knows about being transgender and that age knows how horrible that time is, especially since most can't transition and the body changes.
When we find out that Anna broke up with Shu due to her "crossdressing". It comes off as a sadly realistic problem, and a Tear Jerker since they were so nice together.
And the Tear Jerker goes away when several dozen chapters later we see they've gotten back together and Anna doesn't seem to mind the fact that Nitori is a trans girl
Nitori notices her voice is changing, when a girl tells her that. The look on her face, and the dull surprise, really throws you off. The anime went for a different response, with Nitori smiling and seeming to come to terms with the change, signaling that she's becoming comfortable in her own skin.
When Nitori gets in trouble, and becomes the laughing stock of the school, because she wore a Sailor Fuku to school even though she hasn't shared her wish to be a girl outside her small circle. Takatsuki got off scott-free, but when Nitori does it she gets sent straight to the Nurses Office. This incident also makes Takatsuki stop wearing the boy's uniform.
After having such a nice friendship, seeing Takatsuki's and Chiba's bond break down can be a Tear Jerker. They eventually rekindle though, but it took around three years in-series.
On a similar note, seeing Chiba's dramatic change in personality through the early volumes. She became dramatically more stubborn, aggressive, and depressive.
This happens a lot to Nitori. Her haircut looking too masculine, going to the all-boys school, having to wear the male's waiter outfit at her new job.
If you believe that Takatsuki is confused about their gender due to societal pressure and Chiba's questioning and not in fact cis their choosing to live as a girl and bittersweet shift in both dress and personality from chapter 100-123 can be this. The dead-eyed expression they make when they tell Nitori they no longer want to live as a boy furthers the Tear Jerker quality as well.