Having the same problem. Pre-recorded audio, Stereo Channel inserts work fine. But when it comes to mono inserts only some of the VST’s work. Here is a list of whats working and not.
Eventide Anthology XI - VST 2.4
Eiosis - VST3 3.6.7
Melda - VST3 3.6.5
Plugin Alliance - VST3 3.6.0/3.6.7
Slate Digital (except verbsuite classics) VST3 3.6.6
Sound Toys - VST 2.4
Steinberg - Various
Voxengo - VST3 3.5.1
D16 Group - VST 2.4
Solid State Logic - VST3 V 3.0.2
UAD - VST 2.4
Waves - VST3 V 3.0.2
Slate Digital Verbsuite Classics - VST 2.4
All plugins have the latest 64bit VST 2.4 or VST3 installers and this is a brand new install of Cubase 9.5.10 on a new Windows 10 machine. I have arranged a custom plugin list but have gone back to the default to test and no change. I have uninstalled and reinstalled and gone back to default preferences as well. Did not have this problem on the last computer which was Windows 7/Cubase 8.0.30. Also Installed Cubase 8.0.40 on the new machine and same issue. Kinda frustrated right now. Anyone have any ideas what this could be? Perhaps I did miss a topic somewhere here but I can’t find anything.