MP3 Export Length 3secs too long

Thank you, Marc. I apologize for being unclear. I attached the MuseScore file, but did not attach the 60BPM audio track I was syncing it with, so you would have been unable to test my results.

I suspect you MAY, however, reproduce them.
(1) From the file I attached, export an audio file.
(2) Append one empty 4/4 measure.
(3) Export a second audio file.
(4) Import the two outputs together into Audacity (or comparable analysis tool) and scrutinize how the two tracks align.

When I say the tempo is off - again, in the mp3 output of the file I attached, not in the mp3 output of the file that has my extraneous appended measure - I am saying that, though every possible means (Play Panel, tempo marking and tempo bypass) were used to ensure 60BPM in the exported MP3, the resultant file was not 60BPM, but stretched, to such an extent that I could not sync it to the 60BPM audio file.

Specifically, 33 measures of 4/4 at a tempo of 60BPM (1 beat per second) should produce 2:12 of audio (33*4=132 seconds.) And indeed, after my workaround of appending an empty 4/4 measure, 34 measures did produce 2:16 of audio (34*4=136.) When I say the tempo was off, I mean that the resultant tempo from the file I attached was not 132 beats over a span of 132 seconds at 1 second per beat, but rather 132 beats across 135 seconds, or 1.02273 seconds per beat, or stated inversely, 0.9778 beats per second. Apart from a hearing with the ear, a seeing with the eye* can be found in comparing the incorrect duration of the audio exported from the file I attached with the correct duration of audio exported after my workaround is applied.

My workaround eliminated the symptom of the now-hidden-again error. At this point, my guess is that adding the measure solved the problem by forcing a recalculation of the tempo, duration and total number of beats among all the program variables holding such values. I speculate that the variable data used to ensure midi playback and the same variable data used to export audio are allowed by some execution path to become inconsistent; i.e. that some variable(s) were correctly updated when the 1/16 measure was deleted earlier, and some other variable(s) missed that update. I speculate that, at a minimum, some mathematical aspect of the deletion of the 1/16 measure at the end and the result that deletion should have had on overall statistics of the audio export somehow failed to carry over to whatever relevant software section governs the duration and tempo of exported audio. (Remember that the error existed in the OGG, FLAC, and WAV files, as well.) Whether the failed path lies in the deletion of a measure at the end, or the detection of the deleted measure being not a 1/4 measure, but a 1/16 measure, I could not know. I'm only speculating that the software error is related to that measure being somehow a ghost in the machine. It might lie somewhere else, perhaps in updates generated by the Play Panel or the tempo Inspector. Perhaps it's just a fluke.

Please let me know if I've failed again to answer the question you intended. If this answer still does not satisfy, please be assured that I am trying to be cooperative and am just being inept at it.

Thank you, Marc.

*Job 42:5