Happened to me as well even though i barely bothered to push voice training beyond the androgynous range. Cis people think of their voice as this set-in-stone natural thing and everything else as a form of faking, but a voice is actually just a learned way to use the most complex instrument built into the human body. It's very easy to forget the exact mannerisms you trained yourself for, because that wasn't your natural default voice, it was a learned way of speaking just like the one you learn when you transition.