fwiw, I discovered the same problem with my R4AG flashed with 21.02.2 when WAN is connected to active 100mb port.
https://forum.openwrt.org/t/xiaomi-4a-router-gigabit-edition-internet-not-working/121192/30
FS#3932 reports problem allegedly fixed in snapshot since 20 Feb 2022 for R3G.
Update: I've just installed snapshot for 10 March 2022 for my R4AG. Early indication is the problem is indeed fixed.
Update 2: Also working with 22.03-0-rc1.