While can you edit the MTU, it’s always 1500. I think it’s a bug.
But I’ve had a bug open on this since 18/Nov/21 10:48 PM
SUP-66409: "v7.1rc6 - MTU issues with wifiwave2 ‘Wi-Fi 6’ on Audience
but no response ever* from Mikrotik, dispute repeated pings on the ticket.
Which is annoying since this stems trying working around the lack of another feature, WDS (“4 address mode”) in wifiwave2, by using EoIP (and now VXLAN) tunnels with station mode. But the compressed MTU makes the tunnels less efficient. See http://forum.mikrotik.com/t/using-wifiwave2-to-bridge-two-audience-wirelessly-thoughts-4-address-mode/153357/1
* in fairness, they normally do respond and say something, but why just ignore this I have no idea.