The only possible explanation I can think of is that you were mistaken about it being muted.
Nope. It was definitely muted, still muted when I reopened the project.
I'm now thinking that when I exported I might have selected all the tracks, ie., highlighted everything. I always did that in FCP. That might explain it. Can't think of anything else. I'll do a test when time permits.
PP doesn't offer the option to change which video/audio tracks are visible/muted solely for export. What comes out is what's set in the sequence. Has always worked that way.
The only other possibility I can think of is that you have that same audio duplicated on a second track which you need to scroll down to see.