On fortiswich the same situation. Now I'm stupid , what is going on.
FortiSwitches managed by FG200E on FortiOS 6.4.12.
Also have problem with other devices like UPS management cards (eaton) probably auto negotiation issue and with links between FS and other switches via fiber SFP. But for this I found workaround. HAve to set speed port manually to AUTO (Not 1000Auto).
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