@Viacheslav hello!

More info: Maybe VyOS being affected by https://github.com/FRRouting/frr/issues/18537 ?

Latest RC will work for testing? Ill try a lab here to validate the fix if its available in the RC.

Just to explain this context, usually MSS Clamping is required in PPPoE/ADSL endpoints due to PPP overhead, but for edge routers there is a non common use: DDoS mitigation clouds. They usually encapsulates IP transit inside GRE tunnel and since GRE does not have any alternative like MLPPP (Multi Line PPP) and MRRU (Maximum Receive Reconstructed Unit), the transit session MTU does need a MSS adjust to allow TCP to work well over mitigated clouds.