FortiManager is trying to unset all my global-label

what was the previous version of Fortigate before update?

You cannot have objects with the same name in global as in adom. That's why we add a "g" before any object created in the global db. If you've promoted all of your objects to global with the same names, you're going to have a hard time getting this synced. I'd delete all global db objects and re-promote from adom to global and adding a "g" (no, don't use the ") even then it'll be quite challenging. If you know how to script it with postman it could move things forward faster

hi @rukalmu2 

He means the global-label for the policy itself, after 7.4.x , the global label should be unique for the policy

After more testing, the issue is affecting only policies coming from policy blocks.

FMG does not allow to "Add Section" (i.e. global-labels) inside policy blocks, but this way, when you inspect policies on FGT using "Sequence Grouping View", all these policies will appear as "Uncategorized". To workaround this issue, we used "Edit in CLI" option to add a global-label where needed (trick learned in the past from the support).
After upgrading to 7.6.4, all these global-label are not pushed to the FGT, and the old one were removed. I can manually add a global-label from FGT interface, but FMG will remove it on next sync. This is very annoying, as the "Sequence Grouping View" will miss the groups and become useless.