Walk mode does not work while on ultra mode, sometimes not even for high or medium quality. When already in walk mode (starting in draft quality) and increase the quality to ultra, a bounce occurs, and then falls through the floor. It doesn't matter how thick the floor plate is, it just falls through. This happens to multiple models for me. I'm running 2.5.2.
For me, the bounce glitch happens somewhat randomly in walk mode, like when changing colour or bump setting on materials. It causes the camera to jump up/down, or fall through the floor like you said, or sometimes catapult you tens of meters up in the air. I think it may be related to the 'knee bounce' motion feature they added after 2.5 update where the camera bounces up and down when walking on stairs.
Hey Brookrb and immarkkim , if you both have our latest release installed and this issue is still persisting, kindly send in a dedicated feedback report as described here. This way we'll receive log files and machine information accordingly. I'll also forward this subject to our developers to troubleshoot this issue!
EDIT: Additional info: Our developers stated, that this behavior should be fully resolved with our next upcoming preview, since Sean Farrell did a lot of work on the physics recently (removing the bounce effect among other things) and one thing he did was adjust the physics update to be more stable during hiccups and performance issues, so this should no longer be an issue soon.