I think you will agree with me when I tell you that rendering third-party virtual multitimbral instruments is way more complicated than it should be in Cubase, especially, when creating individual mono audio files. Instruments are considered multitimbral when they have more than just the one main stereo output.
At maximum VST instruments can have up to 16 stereo outputs or a combination of at least one stereo and up to 30 mono outputs depending on the instruments default settings.
Let's talk about Native Instruments Battery Engine Output Settings, for example, which when customizing default to a minimum of 4 stereo outputs when trying to activate mono outputs. This setting cannot be changed.
[ NI Battery preferences settings ]
[ NI Battery activate output settings ]