FLStudio's plugin manager is not up to date with the latest versions of the SDK.
This was taken from the page "VST 3 Locations / Format" in the SDK documentation:
[quote]
On the Windows platform a VST 3 Plug ...
FLStudio 20.6 build 1458, Windows 10.
The "keep focus" option doesn't work with VST3.
I encountered this problem with few VST3 plugins, when VST2 of the same plugin works perfectly.
Example of such plugin is Thorn CM.
How to reproduce ...