Why did the character design change into female the moment mc found out he's a female? What was the purpose of the char pretending to be male until then?
To pretend to be male for various reasons.(could be any number of reasons)
Rhetorical questions that you can Google search if you can't think of any answers yourself: Why does any character have a secret identity? Why would any woman in the series, other series(Mulan) or real life pretend to be male?
I mean just off the top of my head, her pretending to be male dramatically reduces the chance of being targeted, taken advantage of, or assaulted by others or fellow adventurers.
Now that MC knows she is a girl, no point keeping up the disguise with him. Why hide her hair and presumably bind her chest for no reason after being exposed to Auto man?
Certainly when they get back to town/the guild I expect she would put her disguise back on, unless it was damaged or something.(entirely possible frostbite bandages were from chest binding).
Of course that would be where the point of the whole thing should be brought up(when returning). And either conditions would make the disguise unnecessary(example: having obtained a trustworthy adventurer partner/party), forgotten(author has the whole thing having no point from the beginning), or the disguise would continue in public.
I wouldn't hold my breath waiting for this all to be explained, or for her to resume her disguise going forward. I have read too many series to understand that mangaka priorities often are adding more cute waifus. Once the genie is out of the bottle, it rarely goes back.
I would bet 95% she doesn't resume the disguise. Right now I give it 50/50 that the disguises original purpose is even addressed.
