It seemed that the issue was caused by mwan3 itself.
I've tried another device with lessest package installed, below is the test resault:
root@HOME-Router_HB:/usr/sbin# /etc/init.d/mwan3 start
RTNETLINK answers: File exists
RTNETLINK answers: File exists
RTNETLINK answers: File exists
RTNETLINK answers: File exists
RTNETLINK answers: File exists
RTNETLINK answers: File exists
root@HOME-Router_HB:/usr/sbin# ping -I wana 8.8.8.8
PING 8.8.8.8 (8.8.8.8): 56 data bytes
^C
--- 8.8.8.8 ping statistics ---
13 packets transmitted, 0 packets received, 100% packet loss
root@HOME-Router_HB:/usr/sbin# ping -I wana 192.168.120.1
PING 192.168.120.1 (192.168.120.1): 56 data bytes
^C
--- 192.168.120.1 ping statistics ---
6 packets transmitted, 0 packets received, 100% packet loss
root@HOME-Router_HB:/usr/sbin# /etc/init.d/mwan3 stop
root@HOME-Router_HB:/usr/sbin# ping -I wana 192.168.120.1
PING 192.168.120.1 (192.168.120.1): 56 data bytes
64 bytes from 192.168.120.1: seq=0 ttl=64 time=147.649 ms
64 bytes from 192.168.120.1: seq=1 ttl=64 time=143.192 ms
64 bytes from 192.168.120.1: seq=2 ttl=64 time=142.373 ms
64 bytes from 192.168.120.1: seq=3 ttl=64 time=189.098 ms
64 bytes from 192.168.120.1: seq=4 ttl=64 time=256.312 ms
^C
--- 192.168.120.1 ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max = 142.373/175.724/256.312 ms
root@HOME-Router_HB:/usr/sbin# ping -I wana 8.8.8.8
PING 8.8.8.8 (8.8.8.8): 56 data bytes
64 bytes from 8.8.8.8: seq=0 ttl=58 time=293.347 ms
64 bytes from 8.8.8.8: seq=1 ttl=58 time=287.448 ms
64 bytes from 8.8.8.8: seq=2 ttl=58 time=285.715 ms
64 bytes from 8.8.8.8: seq=3 ttl=58 time=287.217 ms
64 bytes from 8.8.8.8: seq=4 ttl=58 time=289.991 ms
64 bytes from 8.8.8.8: seq=5 ttl=58 time=298.135 ms
64 bytes from 8.8.8.8: seq=6 ttl=58 time=286.923 ms
^C
--- 8.8.8.8 ping statistics ---
7 packets transmitted, 7 packets received, 0% packet loss
round-trip min/avg/max = 285.715/289.825/298.135 ms
root@HOME-Router_HB:/usr/sbin#
The mwan3 version is 2.8.2-2.