i have almost the same issue... Fortinet Support told me i should setup
https://docs.fortinet.com/document/fortigate/7.0.0/new-features/766455/dual-stack-ipv4-and-ipv6-support-for-ssl-vpn
but this is 7.0.0 and my fortigate 50E are stuck on 6.2.x anno...
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