Cubasis Problem with MIDI Thru

1. Summary/Title
In order to merge MIDI coming from a backing track on Cubasis with MIDI coming from a keyboard, I have to send the MIDI from the keyboard to Cubasis where it is sent back together with the backing track. This detour is necessary because an USB Hub cannot merge MIDI.

2. Description
I made a dummy (empty) track recieving MIDI from MIDI channel 8, set the track to MIDI Thru, and route the MIDI back through the same MIDI channel 8.

3. Expected Results
MIDI is sent back on MIDI channel 8.

4. Actual Results
MIDI is sent back on MIDI channel 8 ONLY when the whole track (not its contents) is selected. In my understanding, whether a track is selected or not should have no effect at all to any processing.

5. Environment
MIDI is sent from a Studiologic Numa Compact 2x keyboard to Cubasis 3 on a iPad 9th Gen. There is a MIDI backing track in the same project that sends drums to channel 10. There are no other applications on the iPad running which could affect MIDI.