One audio file out of many failed to export

Pr Version 14.5.0 (build 51)

Me Version 14.5 (build 48)

Windows 10.0.19041 Build 19041

Dell XPS8900 x64 i7-6700 @3.4GHz, 4 cores, 8 logical processors

RAM 32GB

Dear all,

A very odd situation last night.

Not sure if this is a Pr, Me or system problem but it's never happened before.

It's a 50 minute timeline with mulitple MTS, MP4, M4A and MP3 audio files.

The 1st H.264 export was successful (although I increased the volume of an MP3 (on A9) and M4A (on A10) at the end of the timeline. No other changes were made before 2nd export).

The 2nd H.264 export: audio of an MTS (on A5) was silent, although other audio clips on A5 were fine.

The 3rd H.264 export (with no changes made at all since the 2nd) was successful!

There were no muted tracks, nothing had disappeared from the timeline and no changes made between second and third exports. Everything was proofwatched the first time (by someone who spots everything!) The silent part was only noticed after it went public.

I wondered if this is happening to others (but can't find anything similar posted) or just something we have to be careful of happening without warning. At the very least it's a reminder to me to proofwatch everything!

Any help would be most appreciated. Very many thanks.