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 ...
FL internal plugin (say, Sytrus), I can work on the plugin interface, click on another element (the piano roll, for instance) and this brings the other element on the foreground. This doesn't work with external plugins that I have ...