You are absolutely right, that is what was happening. The eth0 MTU was 1492 and pppoe-wan 1484.
I deleted the option and now everything is correct, eth0 1500 and pppoe-wan 1492.
Regarding why it broke delegation and not something else, I have no idea. Maybe it broke something that went unnoticed. And it was my wrong to use it without checking first documentation or the "ip link". Too much confidence I guess and I got carried away from other operating systems.