You appear to be asking for support on a prerelease version of Windows.
We do not provide official support on those as they are liable to break things at a moment's notice and features in them may never ship in a released product.
In this case it sounds like there has been a tweak to where taskbars can be positioned but Start11 which is designed for released versions where this is not allowed is blocking this though I cannot say why it would be as assuming you have turned off the enhanced taskbar features it wouldn't normally be interacting with that.
In short, thanks for letting us know, we will investigate to ensure it is resolved before this feature is released.
Start8 and 10 will not work on Windows 11.
One final really dumb question, does it actually work if you temporarily uninstall Start11?