I've had a few thoughts since posting. Will check tomorrow morning US eastern time) and see what it gets me. Thanks for the reply.

Model and Layout project created in 2017. I opened a foundation plan in LO to update changes made in SU model. All dimensions were red. Right click to reconnect. Still red. Got rid of red by going to "View" and disabling warning. Render Viewport and everything created in SU disappears. Gone. Change scene reference, that scene appears. Select the scene I want, nothing. Expanded the viewport to see if it relocated itself- nothing. I have been following Nick Sonders method of using a Vector rendered viewport for dimensioning and that's the one that disappears.

I'm baffled.