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 as VSTi in other DAWs.
And found some issue with FLStudio in all DAWs I tested.
When I used FLStudio as VSTi and played the project,
the MIDI notes of FLStudio VSTi track at start ...