
I think its also because they're nobles. In these royal isekai mangas, some FLs do express that cutting hair is like a symbol of falling from grace when you're a noble, one of the most physical indications of shame. One isekai manga where the FL wanted to become a commoner, she briefly mentioned how her short hair is seen as of commoner status and that she didn't mind crude comments about it//whereas if it had been her original, noble (non-isekai character) self, they'd be insulted.

I may be incorrect but it sounds like the manga "Lady rose wants to be a commoner"
http://www.mangago.zone/read-manga/lady_rose_wants_to_be_a_commoner/
okay but like... why do ftm crossdressers in manga always keep long hair under a short wig....... wouldnt it make more sense to cut it short and have a long wig for 'girl' occassions??? smh