(Regression) Adding an instrument adds twice its mixer channels

Using current 2.2-dev 072c135

1. Create a new score. (You can use a template or chose instruments).
2. Add instruments (either from the wizard or add them to a score created with a template)
3. Open mixer

Result - Two mixer channels are assigned to each added instrument for each channel needed. Adding a string (i.e. viola) will add 6 channels rather than 3.
Expectation - One mixer channel should be assigned to each instrument for each channel needed

Note: I opened the score in version 2.1 and the extra channels were still there. This only happens if you add an instrument either through the new score wizard or the Instruments dialog. Templates (that I tested) add instruments as expected.