We have multiple models of Fortigate in our environment. We only just started experiencing this on a 90D when upgrading to v5.2.6.
I called FortiNet, they confirmed the bug ID (0300588) listed in this article will be resolved in 5.2.7. There is no public article about the bug from FortiNet. I have escalated my case with them as I was told the higher tier of support can release some information about the bug to me.
I have a 300D and 311B running same firmware version that does not experience this issue. FortiNet told me this is not a model specific issue and obviously it isn't a firmware specific issue as multiple devices running same firmware don't experience the issue.
Given what I'm seeing I'm not so sure this is without a doubt an issue on FortiNet's side of things and not Solarwinds.
We're running Network Configuration Manager 7.4.1.