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 ...
FL 11 easily and can save the project and open it in 12 and no problem, however cannot add it permanently.
I can find it with deep scan and I have renamed it to something different and yet it is still ...