I observe this issue on [[https://openwrt.org/inbox/toh/xiaomi/xiaomi_mi_router_4a_gigabit_edition|Xiaomi Mi Router 4A gigabit edition]] with OpenWrt 21.02-rc3 (r16172-2aba3e9784). The WAN to LAN download speed is about 20 Mbps on the 100 Mbps channel, while the upload speed is OK.
I am running 21.02.0 on MT7621 device.
DHCP connection over 802.1q VLAN - 1Gb/s with 0% sirq.
PPPoE connection over DSA wan port - 100Mb/s with 20% sirq.
Looks like HW offloding works for VLAN and doesn't for PPPoE.