I had a rocket with a sugar engine, oxidiser tank, rover module and nosecone. I was able to build on the inside of the nosecone. I added and then removed a trailblazer module, and all build commands on the inside of the nosecone were listed as unreachable builds.
When I added the trailblazer module back, then suddenly I could build in the rocket again.So it seems if the rocket is ever made shorter, then duplicants cant enter the nosecone.
Reduce rocket height below it's previous max height, and duplicant appear to be unable to enter the rocket. Add a module to the rocket, dupes can then enter again.