Since version 4.0.x the TX bytes counter of the WWAN module shows a weird value.

Example:

$ > ifconfig wwan

wwan0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet xxx.xxx.xxx.xxx  netmask 255.255.255.252  broadcast xxx.xxx.xxx.xxx
        inet6 xxxx::xxxx:xxxx:xxxx:xxxx  prefixlen 64  scopeid 0x20<link>
        ether xx:xx:xx:xx:xx:xx  txqueuelen 1000  (Ethernet)
        RX packets 19036  bytes 19190321 (18.3 MiB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 15874  bytes 43228847574631 (39.3 TiB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

39.3 TiB - wow, absolutely not true

The WWAN is used as bridge to my internet provider (LTE usb stick)

I use the counter to control the traffic. It's only the TX counter, the RX works ok.