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I Had the same issue
What I did:
1. On the LAN interface I left both MTU/MSS empty (using default MTU 1500)
2. On the WAN interface I used: MTU=1492 (using PPPoE connection) and MSS=1492. According to OPNsense help, having MSS set to that value will clamp MSS both for IPV4 and IPV6. For IPV4 it will be MSS value minus 40, and for IPV6 it will be MSS value minus 60 (1452, 1432 correspondly)
For me that solved that issue.
What I did:
1. On the LAN interface I left both MTU/MSS empty (using default MTU 1500)
2. On the WAN interface I used: MTU=1492 (using PPPoE connection) and MSS=1492. According to OPNsense help, having MSS set to that value will clamp MSS both for IPV4 and IPV6. For IPV4 it will be MSS value minus 40, and for IPV6 it will be MSS value minus 60 (1452, 1432 correspondly)
For me that solved that issue.
