Quick update, I might have been able to reproduce the issue ( GreasyPenguin14 working "offline" is a great hack to accelerate debugging this issue, thank you!)
It seems it is related to the known and very annoying Python forking issue (and this is why changing to "spawn" method solves the issue):
https://bugs.python.org/issue6721
Long story short, in some cases when forking (i.e. ProcessPoolExecutor), python can copy locks in a "bad" state, this means that you can end up with a lock acquired by a process that died, from here it is quite obvious that at some point we will hang...
I think the only way to get around it, is with a few predefined timeouts, so that we do not end up hanging the main process.
I'll post here once a fix is pushed to GitHub for you guys to test