No luck, thanks. But here's another project with the same problem, and I just noticed that in both cases, the stairs don't appear in 3D Docs with Renovation: Proposed, but they do appear in Renovation: Existing. See attached.
AC fan since v 7. Currently on AC 26 Build 5003 USA FULL Apple Silicon,. 2022 Mac Studio, 32G ram. OS X 12.6.2