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 Version 12.5 [Build 59] - 64 Bit
OS: Windows 10
It seems to partially ignore the UI itself, I tried some of Meldau Productions Plugins,
and the free version of the Surfer EQ, and the demo version of Zebrify by U-he.
Also ...