After much fiddling, I finally found the problem. (I REALLY wish there was more knowledge widely available online on AUv3...)
It seems that both Logic Pro X and Garage Band on each render cycle ask the plugin process for blocks of different lengths depending on whether the plugin is in the selected track or not. If the track is selected, the requested block will be the length set in the DAW's settings (I/O Buffer Size), presumably for highest priority rendering? Unselected tracks are asked for 1024 frames (the longest Logic's buffers can go, it seems), regardless of I/O Buffer Size setting.
1024 frames is longer than the
AUAudioFrameCount maxFramesToRender = 512;
the Audio Unit Extension template stubs in DSPKernel.hpp, thus making it so that rendering fails only when on an unselected track. (the short period glitch noise I mentioned appears to be whatever values were left in the output buffer since the last playback being re-output once every 1024 frames)
Setting maxFramesToRender = 1024; fixes that problem.
And now for a heavily opinionated rant:
I can't help but feel this default maxFramesToRender value is setting newbies (like me) up for failure, since 1) it's never mentioned in the official tutorials or documentation (AFAIK) 2) it doesn't play nice with Apple's own DAWs, presumably the most obvious places to test 3) it initially works, but only up until you try to play two tracks at once, at which point you probably already have a lot of code down and are all the more susceptible to confusion.
But oh well, I guess it is what it is.
