I almost exclusively use Colemak-DH and have 0 issues typing in QWERTY when I have to. I chalk it up to my phone keyboard still being in QWERTY so I get enough exposure from that to not forget the locations of the keys (it's certainly not as well retained as it used to be, I no longer remember the location of all the keys in QWERTY from memory). I do use a columnar split keyboard though (Moonlander) so it's possible the radical difference in how I use the keyboard helped to keep the muscle memory separate.

It's like driving stick vs automatic.

I don't know how it works, but when I switched over (20+ years ago) I was dysfunctional in both qwerty and dvorak for a good couple of weeks if not a month, then slowly got the hang of it. I use qwerty now at work and dvorak at home, except that I've switched the main keyboard layout at home to qwerty and just use dvorak for programming and anything in a terminal (on linux), which is where I spend a lot of my time.

I can mentally switch over to dvorak and program for a while, mentally swap back, and type normally for other things. I mainly switched the default layout back to qwerty because I was tired of remapping all the keys in every game I play. Sometimes when I come into work on a Monday morning I'll type gibberish for a couple of sentences then mentally flip back to qwerty and be fine.