This is a Windows 24H2 bug.
Workaround:
vivetool /enable /id:50557073,52110492
https://www.elevenforum.com/t/kb5041865-windows-11-insider-release-preview-build-26100-1586-24h2-aug-19.27764/post-490811
So just disable transparency in StartIsBack and let AeroGlass handle it all. Then when you unplug the start menu will also be solid. On a seperate note I have noticed a couple of issues running the latest beta on Windows 10 Tech Preview. The right click menu on the start button doesn't work and the app list font is all messed up.