I had similar problems but with older versions DanceXR-Mix-Pro. For resolving this problem, when start app i looked down on floor near my position, this will be position where models will be appear on load. But i noticed that this problem was resolved in newest versions and now no neccessarity to do any action on start app. If you have this problem with latest version try to reset settings (Envirement -> System Presets -> Reset All) then restart app, probably it can help.
I also use Quest 3 (with custom models)