On 13.1, seems we may have an issue with project page layouts created in previous versions.
Have a custom layout created years ago, where we can add project tabs, but the new tabs won't display on the project. If I create a new page layout and add the same tabs, they show up fine.
We have a workaround, in that we can create a new layout, with new tabs and move forward. However, would prefer to upgrade our existing page layouts, if possible.