This has since been patched, so no more T-posing Dante I'm afraid.
Patchnote from Capcom: / 1237062238017671169
In case you somehow still have the issue, then here's a fix for it:
HOW TO FIX THE GLITCHED SAVEFILE: • DMC3 Switch - How to Fix T-Posing Dante
Join Dante through his latest adventure as he stylishly T-poses through all of Temen-ni-gru.
Shoutouts to Dinocrusher for being MVP in researching the phenomenon. For more information, check out his Twitter thread: / 1235334388566708226
Edit #2: Nevermind, this bug just happens seemingly at complete random if you're playing as Vergil. We have no idea if there's a way to avoid it. Maybe by making multiple savefiles, but that shouldn't be our responsibility to make sure of.
Edit: So this bug seemingly does just happen when you load Free Style Dante into Co-Op with Vergil as player 1. Kind of a big oversight there. Only do what I have done with a fresh new character. Everybody needs to be able to assert dominance, after all.
I have no idea how this bug happened. I had played Bloody Palace a couple of times prior as Dante beforehand, and then simply went into co-op after having beaten the game on Hard as Vergil. So much for 17 hours of playtime grinding orbs and style experience. On the bright side, I can at least move on to new things now.