FLStudio has always allowed the use of dual (or more) monitors but it may looks as if it is restricted to a single monitor. To use multiple monitors there are 2 options :
Set the window you want to move ...
FLStudio is open. Any monitor changes being made should be done while FLStudio is closed to avoid this issue.
Solution
In order to recover your off screen plugins you must perform a manual reset of your FL ...