I am seeing a massive difference in the traffic reported on my guest interface on the router vs dumb access point and want to report the bug, just not sure against what component.
My setup is a router and dumb AP with VLANs are separating several interfaces. The guest zone is exclusively used by WiFi so I would think these values was be close.
As you see from the screenshots, the AP has about 24 h of uptime on the interface vs 10 h on the router but if anything, you'd expect that to influence the numbers in the wrong direction.
It is likely that the local VLAN interface on your AP (br-lan.3) simply sees no traffic because it is bridged through from wifi to the ethernet port without delivering frames to the CPU
If it helps, trace how the bridge gets setup, check what tools like ifconfig (also iw and ip) show (i.e. does the wlan if show consistent TX info with the bridge if you check with ifconfig) and then try to follow where luci gets its information from.
To find the package of a file related to a bug use opkg search: opkg search /usr/sbin/brctl
for me returns busybox (likely not what you'll need)
Since it's wifi you may have to go down to mac80211 (kernel and/or things like opkg search /lib/wifi/mac80211.sh. Which is why this caught my attention.
HTH
Edit i'm also curious about the device and build (i.e. master or a stable version) - I have a somewhat similar vlan/guest network config.
