Hi Rudy,
Thanks for your reply. The map setup is fine. I already packaged the project a few times before in 4.15.1.
I tried removing the intermediate, build, binaries and saved folder and re-generate the project, same issue.
I also migrated my maps and assets to a new clean project, same issue.
I think somehow some assets got corrupted for whatever reason and not specifically by the engine. I did make a copy of the project for a different feature test, which does build correctly (but missing my latest changes). So for now I will add my changes to that version and continue.
My lesson from this is of course to use version control…