MTU on Etherchannel links

For Layer 2 EtherChannel, only the interface MTU can be adjusted. You must change only the interface MTU of the port-channel interface. Any change to that MTU will automatically change the MTU of all of the physical interfaces, if they support the configured MTU. If you are using a switch that does not support individual MTU configurations per port, you must use the system mtu configuration, which will change the MTU on all ports of the switch.

For Layer 3 Etherchannel, the individual physical port IP MTUs should not be modified. Only the IP MTU of the port-channel should be modified. If you modify the IP MTU of individual ports, the IP MTU of the port-channel will inherit the smallest IP MTU configured on any of the physical ports. This will cause problems on your link as some physical channels may fragment IP packets or may drop them.

https://networklessons.com/ip-routing/pppoe-mtu-troubleshooting-cisco-ios