On pppoe links, when you enable to change tcp mss mt adds mangle rules for both incoming and outgoing traffic so mss is set to whatever value (eg: 1480).
The problem is: what if an incoming packet had a lower mss value (eg: 1400) ?
Does mt overwrite a 1400 mss with 1480 ?
Maybe mss rule should have a “tcp-mss=1481-65535” condition?