Can anyone tell me why, when a second MIDI interface is connected, BIAB allows MIDI data from it as well as the primary (or selected) midi driver?

It should ONLY receive midi data from the device selected as the MIDI input, regardless of how many other devices are connected. Especially when the other device is set on a channel that BIAB is not using = example 16, or even 15.

Is it because BIAB programmers are relying on the Windows class compliant MIDI drivers, and have not actually written their own?

Perhaps it is a question for the BIAB programmers?