This specifically applies to the current beta versions of LMMS including and since 1.2rc5.

Some VSTs are known for having issues with the GUI or other functions. One thing that helps is if you turn off VST Embedding. This can be found in your general settings for LMMS.

Some synths which I have tested that work better in No Embedding mode (as opposed to embedded) include:
Kontakt 5 Player (Demo Version)

All of which were tested in 1.2rc6 Windows 10.

Just whatever you do—do NOT close the windows by using the X tab in the corner of the window. Just minimize them or close them using the “Show/Hide GUI” button in VeSTige!

This isn’t a guaranteed fix however. Most demo and cracked versions of commercial/paid VSTs will have code segments which prevent full functionality until the full version has been purchased.