We are seeing this issue with FortiClient 7.0.6 and 7.0.7, both the free and paid/licensed EMS version, on some of our endpoints. It seems like an endpoint becomes 'cursed' and after this point, no amount of clean uninstalling/reinstalling will fix it.
I have tried doing a full uninstall, FCRemove, running the WAN Miniport repair tool + reinstaller, and a clean FCT reinstall and the issue still happens. I was just connected on a test device and it started dropping at 20 minutes, then 15, then 10, then 5, then it was almost immediately.
We've seen this on various versions and editions of Windows at various states of patching/feature upgrade installation. Never had this problem until 7.0.x. We've updated chipset, motherboard, and NIC drivers on them as well, with no change.
Does anyone have any ideas? I have a case open with TAC but historically the troubleshooting of these issues has been so slow that we ended up wiping the device and starting over - interestingly, on one device the problem came back even after that! It seems to be mostly Realtek network adapters that have this issue.