It seems, Cubase won’t allow me to set stereo plugins to mono in the channel editor/routing tab. As channel configuration there’s “Standard - Stereo” preselected with no option for a mono setting.
It works with mono plugins only in 8.5.0 as a workaround.
Loading an insert preset from 8.0.3 with stereo plugins loaded that are set to mono will work though. So the ability is still there, just the chance of altering the setting is gone. In that scenario (loading such a configuration from 8.0.3) I can set the mono routing to stereo but then I’m stuck. There’s no way back to mono anymore.