I am sorry to have to report a new bug.
I have SPPB pages that were built in SPPB 3. I migrated them to SPPB 5 yesterday by exporting and then importing. Luckily I backed up the original site by copying it to a development site (details below), so the resulting pages can be directly compared. You will see that in migrating one of the pages, links to a bogus page (/coming-soon) has been created. I did not do this - it happened in the automated upgrade progress.
You can see the links by looking at the page source. The rest is in the hideen content section.