I am (still) trying to troubleshoot phantom data usage on my wwan/qmimuxN interface, seems to be a failover / mwan3 problem. While doing so, I found that the darkstat package will not start properly when configured for a non-default interface such as qmimux0.
Since nobody has replied to this, before the topic is auto-closed I will just say that this has been working well for me, hopefully the change will be eventually baked into the package directly.
darkstat works fine on recent openwrt versions and a non-default interface without the workaround. The basic scripts from /lib/functions/network.sh are identical, the cause must be somewhere else.