Why do I get cannot find a Qt installation in "/some/path"?

I have compiled Qt 6.8.2 from source. I have a pure debug build in /data/memmenlauer/bda/usr-dst-U2404Sk64c201/Debug and a release build in /data/memmenlauer/bda/usr-dst-U2404Sk64c201/Release. To be super clear, I therefore have i.e. the file /data/memmenlauer/bda/usr-dst-U2404Sk64c201/Debug/lib/libQt6Core.so.6.8.2 available.

When I set the following, I get a warning:

"qt-core.qtInstallationRoot": "/data/memmenlauer/bda/usr-dst-U2404Sk64c201/Debug"

The warning is Cannot find a Qt installation in "/data/memmenlauer/bda/usr-dst-U2404Sk64c201/Debug".

I checked the extension logs, but there is no hint as to why this should not be a correct and fully working Qt installation. I do set some Qt cmake settings to disable some features I do not need - but all the general tools and commonly used libraries and features should be there.