With all this sheer speculation in mind, take a look at band 4. Try reducing the Q factor to 5 and see what happens.

When the Q factor is below 1, there is no distortion. The distortion slowly increases as I push it past 1.4 or so.

This doesn't indicate a bug, since that's a bass frequency and I'm reducing other lower bass tones by decreasing the Q.

Although I will mess with the other settings more and see if there's a particular one that does it like an on/off switch. Good idea...