MIDI notes not sustaining for full duration

Cubase 8.5/Windows 7

MIDI notes on certain VST instruments are not sustaining during playback. They will sustain in real-time, but when using the transport to play, they are dying early. Any thoughts on how to remedy this?

Edit: in other words, instruments with infinite sustain are dying for no apparent reason.

Got a video via dropbox or more details?

If this is from a MIDI source, have a look in the list editor for any instances of CC64 or CC120.

There is nothing in the list editor aside for CC1.

Would a video actually help? It really looks very simple on the screen:

Transport hits the midi notes -> audio activity in the meter (no bussing) -> audio activity sheepishly dies away while the transport continues to go through a held MIDI note (both synths - serum and omnisphere - are set to long sustains)

The CPU meter isn’t spiking. No volume automation either…

Not that it brings us any closer to the solution, but do these notes die away gradually, or simply cut off?
If the former, does the release parameter of the instrument alter this “die away” time?
Do you have any other MIDI track accessing the same instrument (on the same MIDI channel)? If so, try using in Solo.
Also, check, in the List Editor, that the notes don’t have duplicates (possibly, with the “other” note ending where what you hear starts to decay).