Dual-WAN traffic 35.3MB/hour but not active

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I am using a LTE USB Stick (Huawei E3372) in fail-over mode on my AC86U. It always worked like a charm for months.

Past 2 days it started generating 35.3MB traffic per hour without actually being active or the primary WAN failing.

In fact I disabled Dual-WAN yesterday evening and it still generated 35.3MB/hour until today so I removed it for now.

I upgraded to the latest Merlin version maybe a week ago (without the need for restart it seems), so I don't think this coincides and if I understand the changelog there is nothing regarding Dual-WAN that changed.

Before I reconnect the USB stick and enable traffic analyzer (does it actually show WAN0 and WAN1 separately?), does anybody have pointers how I could debug this or what I'd need to look at?

I am logging to an external syslog server, but I didn't see anything suspicious but then I am not very proficient with fail-over or USB modems. Maybe the USB Stick is doing something on its own.

Thanks for any pointers. I am paying for traffic on this LTE stick, so I can't ignore it.

PS: The Huawei E3372 is in stick mode it seems if this matters, there is no web interface on if connected to a computer.
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