For anyone else that might have the same issue and has stumbled across this thread, the solution that worked for me is:   On the remote Fortigate side, change NAT Traversal to Forced. I previously had it on enabled. After changing it to forced, my tunnel stabilised and didnt go down. ... View more

Hi   I have this same problem. If you mean subnets when you say segments, they are matching on both sides. Do you have any other insight on what can fix it? ... View more