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 ...
Studio One Pro V3.0.2 along with Harmor and many other Image Line VST plugins. I have found a limitation that currently prevents me from using recorded audio from Studio One in Harmor.
Drag+drop FROM Studio One TO Harmor VST does ...