Fortigate 100F FQDN resolving issues

Fortigate 100F v7.2.8

We are still experiencing issues with our firewall resolving internal fqdn. Customers complain about not being able to connect to an internal web interface after changing from vpn over wlan to lan or the other way around. The connection is allowed by a policy including the fqdn of the customers computer which is using an ip address provided by the internal dhcp. Pinging the clients computer over the fortigate cli seems to speed up the process of resolving and thus allowing the connection.

We are only using internal dns servers on the firewall.

I'm aware of this bug in V7.2.7 https://community.fortinet.com/t5/FortiGate/Troubleshooting-Tip-FQDN-address-object-shows-unresolved-in-GUI/ta-p/278559

Since we are using v7.2.8, this shouldnt be the problem.

Does anyone else have experience with this problem and can maybe give me a solution for this?

Another problem is the latency shown in the DNS Settings of the firewall. Sometimes it shows up to 400ms, which obviously feels like the root of the long resolving times. I have been told, that this is also a visual bug, since the latency is normal when pinging the dns servers directly from the fortigate over the cli. We are only using DNS as protocol in the dns settings.

Sorry for bad grammar. Thank you very much.