Cannot export audio tracks if I select 'multiple channel selection'

If I select ‘Multiple’ and choose one or more audio tracks, nothing is exported. A wav file is created of an appropriate size but there’s nothing in it.
If I include Stereo Out in the multiple selection, then the Stereo Out wav file contains the audio tracks, but the individual exported files for the audio tracks are blank.
If I solo the audio track and do a single channel export of Stereo Out it contains the audio.
If I do a single channel export of just the audio track, it is blank.
If I select VST channel tracks in the multiple track export, they do export correctly.
The output routing for VST tracks and audio tracks is the same - Stereo Out.
I can hear the audio tracks in playback within C12
I’ve had a good look on the forums and can’t find mention of this elsewhere, so I assume it’s just me…
It only does this on one project, so I don’t think it’s an issue with C12 per se.
Anyone got any ideas please?