Annoying it is. So I did some testing. (1.17.02.45)
I can confirm the reproduction of this bug.
Not the first channel : confirmed. Only >= 61 (just 61-64 in this case) : confirmed.
I used an XM-file to open, so the max channels is 64, and it happened even without needing arrow buttons to scroll.
It appears to be not only with switching from the pattern tab, but I guess with all others as well. Here after the sample tab:

Remedy so far : Some refresh of the general tab representation. For example after a minimize/maximize action, the mix is gone. Or switching to another application on the foreground, then back to MPT.