Thanks for the real-time audio mixing suggestion, I hadn’t previously been aware of this.
It didn’t fix all the rendering issues, however. The audio is unchanged from the original Ex1 footage; it’s always worked without needing rendering in the past.
The issue is very strange, a clip I’m looking at now is demanding to be rendered if I put a cross fade on the left channel of audio, but doesn’t need rendering on the right channel, even though it’s audio from the same source.