a novel sound bug/issue/snafu

Rendering an edit I have music on track one, set to type: music. Generated keyframes, nothing else applied.

I have mastering switched on with a low pass and "make room for vocals". This is the setup I've used for years.  3 hours later I'm still trying to get it to export.

It's not rendering track one.  I've tried:

a different wav file in track one, that didn't work either - so the issue is probably with the tracks. 

ditching cache and cached files, render and replace, I've ditched track one completely. Track 2 appeared to work fine, even with everything copied to it exactly - tried a small section - it worked. Rendered the whole thing and not only did it render broken - with something that sounded like ring modulation and weirdly warbly - but the track in premiere subsequently sounded like that too. Wth? The individual files sounded files sounded fine.  

I have tried switching the audio hardware to "no input" (but grasping at straws because other music in the edit renders fine). 

OK so just about to post this and I got a successful render by moving everything to another track. I have deleted the original track one, but it appears that premiere hates the new track one, too. 🤷

Now the question is whether I need to worry about this or just rename track one as "closed for maintenance" in case I need to return to the edit.