That's strangely low. Without tweaks, I would expect MTU 1492 for a PPPoE interface (1500 - 8 byte PPPoE header). You're not running a VPN or anything like that over it?
Also, how are you testing? Using Linux:
ping -Mdo -s1472 x.x.x.x # => MTU 1500 (20 bytes IP header + 8 bytes ICMP header + payload)
Apologies! Upon investigation PPPoE link MTU was weirdly set as 1480 on the router i/f and should have been set as 1492 for the PPPoE link. With that said ping testing to 184.108.40.206 with a 1464 MSS/payload gives the expected result of 1492 on the PPPoE uplink.
Via IPsec the MSS/payload is 1398 or an MTU of 1426.