Bendy is the default for all Workshop maps, regardless of how they are made. The game itself switches to some kind of different "mode" when you play workshop maps. It's the same mechanism that fills in the chamber sign with the map number, title and hazard icons.
I have changed the model back to Chell in my yet to be released map and I haven't noticed any feet sliding. I'll have a closer look some time.
Edit: Wow, that actually looks pretty bad! How the hell did I miss that?
It does get fixed if you set sv_use_bendy_model (or something like that) to 0, but that only takes effect after restarting the map...
I guess the models use different names for their animations. Time to decompile those suckers and see what those differences are...