I am connected to Sky FTTC (80/20) using a Vigor 130 bridged to an Asus AX88U (1483 Bridged IP LLC, VIP:0, VCI: 101, Multimode, MTU 1520) and the connection is very stable with upstream & downstream SNR of 5 (dB). MTU on the router is set at 1500 (pinging with a buffer of 1472 is the maximum before fragmentation).

I seem to recall that when I first installed the modem the default MTU (and the Max MTU) was set at 1500, but as noted above with the current firmware it now shows 1520 (as does the Max MTU), which is something I don't recall changing.

While I don't want to break anything that seems to be working so well, is there a reason that the default MTU has been increased and does it matter that it no longer matches the MTU on the router?

MTU was changed to provide for Jumbo Frames , you can set whatever you want / need.