I've gradually unplugged everything on the network side of the Mikrotik without seeing the Tx usage drop. This suggests to me that the Mikrotik is doing something stupid with the connection. Or perhaps someone's compromised the router?
This is torch on the interface. Not sure how to get info on the source/dest port?
As far as I can tell the bandwidth test is not configured at all (there's 0.0.0.0 in the "Test to" field.