I ran into this on some FGT too. And I also openend a case wth FGT TAC.
They confirmed to me that there is known issues with SD-WAN healthcheck causing the behaviour I saw and the threadstarter reported. This reported in issue #576646 and #583247.
This is a 5.6.11 only issue accoarding to TAC.
Their solution is either to roll back to 5.6.10 or to upgrade to 6.0 or 6.2 even....
You can imagine that (as I have 20FGT to roll back or upgrade that are in productive use) I am currently not really excited :\
This is pretty annoying to me :\
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