Hi kronicklez,
Please refer this KB, MTU calculation for IRB interface is little tricky.
https://kb.juniper.net/KB27446
MTU on physical port may match but there are other consideration too which you can check in above article.
For troubleshooting please check following:
#1 please check irb inet mtu on both side
show interfaces irb.x | match mtu
#2 please ping the loopback of other router and source it from local loopback. If you think physical MTU is 1518 I think you should be able to ping with 1472 packet size (18 ethernet + 20 IP + 8 ICMP). If ping fails at 1472 find out the path mtu by decreasing the size
ping x.x.x.x source y.y.y.y size 1472 do-not-fragment << try it from both side
PS: If my response solve your query please accept this as solution, kudos are appreciated too !!
Thanks
Vishal