Several months ago, you had to config-tweak HW offloading on Mediatek mt7621 devices, to properly work in some corner cases.
in /etc/sysctl.conf add:
net.netfilter.nf_conntrack_tcp_no_window_check=1
see also How to check if hardware NAT (flow offloading) is enabled? - #8 by Pico (there were some more older threads about "conntrack")
In the past (and probably still today) the HW offload checkmark in LuCi would not take care of that additional config file setting. But I donno, if this is still required to do or how mt7622/mt79.. behaves.