Ah apologies for not being clearer - but it wasn’t just deleting the .git folder that fixed it. Just doing that would result in the same issue, an old ID would get picked up from somewhere after a second resync.
The breakthrough came when we simply changed the name of the experiment under “Experiment Settings” to something new and deleted .git and created a new project. After that, it kept its new project ID and didn’t change to the old one on subsequent resyncs. That’s why we were wondering if it was something picked up from gitlab based on the old name and from an old project that no longer visibly existed (i.e cached or not fully deleted).