Couldn’t find this issue via search.
When I open a project that has an external instrument, which is in-fact turned off, Cubase will give me the “Realtime mixdown” required notification when I export a mix. If I go to the VST instruments and turn the offending instrument “On then back Off” the problem is solved. It seems like the “on/off” state of the external instrument is not being validated at startup. Has anyone seen this problem. Not a show stopper just annoyingly inefficient.