Beat is off in final .wav mixdown

Hello,

I'm using Qtractor 0.9.24.15 and Qt 5.12.8

My song is finished and I'm happy with the way it sounds when I play it in Qtractor.
I mixed down my song to a single .wav file, from Edit -> Track -> Export Tracks -> Audio.

It finishes saving the .wav file.

However, when I play the .wav file, the song does not go from one bar to the next smoothly, there is a slight (but noticeable) pause, then the next beat starts.

If I can describe the beat as text, it should sound like this:
beat beat beat beat | beat beat beat beat | beat beat beat beat | ...........

But it sounds like this:
beat beat beat beat | (0.3 second pause) | beat beat beat beat | (0.3 second pause) | beat beat........

It sounds perfect in Qtractor, just not when it's mixed down.
I know I can bounce the playback to one track and use that to export to wav but I would prefer to mix it down to a .wav directly from the Edit menu.

Any ideas why the beat is off in the final .wav but not in Qtractor?

Thanks,