So @quarky is absolutely right that it looks like PMTU issues and makes sense giving that we both are using PPPoE. Nevertheless once again I don't understand why wouldn't it break since offloading is applied after connection params negotiations.
Can you try to replace the rule I suggested previously with those two? The difference is that those will skip offloading only on first three incoming and first three outgoing packets. It's not a big difference from a computational point of view, more out of curiosity.
iptables -A forwarding_rule -m conntrack --ctstate RELATED,ESTABLISHED -p tcp -m connbytes --connbytes 0:3 --connbytes-dir original --connbytes-mode packets -j ACCEPT
iptables -A forwarding_rule -m conntrack --ctstate RELATED,ESTABLISHED -p tcp -m connbytes --connbytes 0:3 --connbytes-dir reply --connbytes-mode packets -j ACCEPT