That's a trade mark of Watanabe Asia though, when I first started reading her work I found it extremely weird but now if it doesn't happen it's not her work somehow ╮(￣▽￣)╭
In this particular manga it seems like she might have taken some year breaks between chapters & in that time her art improved to the point of being almost irrecognizable from the where she started, but if she's going off-model on purpose, then that's just unprofessional.
It's a bit of both to be honest, but to me it seems more like a personal preference of her according to her work. If you see the sequel (the Mitsubana DJ) she uses the more relaxed style- the latter one used here as well. I feel like more than her being lazy or unprofessional, it has a lot to do with the storyline and writing. There's a certain point in the story, when things are finally set free that might influence the art change if it makes sense at all? That's the feeling I get from it having read most of her work by now.
But at the same time, it could be her just going with what she wants or going off-model like you said, I can't know that's simply the feeling I get :)
It was so funny how the uke actually started out looking semi-manly but progressively with each chapter he looked girlier & girlier & even became shorter & thinner lmao
Overall, i get what the author was going for, but it was poorly executed. If you're going to write something messed-up commit to it at least instead of half-assing.....