Importing a track from 1 Audacity user to another and finding a speed variation in the tracks

I am using Audacity 3.4.1, so is my friend. We regularly work on projects together, we will lay down a guide track which one of can then build content onto by adding additional tracks. We save the Aup file in a shared folder in DropBox so he can access it. He then adds his content. He has always found issues saving his updated content into the shared folder, so the work around has been to export his track to mp3, I then add it to the working project and all’s well. At least has been until we recently upgraded to the current versions! For some reason the exported track he provides me with is now shorter (quicker) than the other tracks, I wondering if anyone has an explanation for this.
Let me add that we are novices with Audacity and will probably be doing many things wrong, but our process has worked well enough up until now.

I assume it’s a slight difference?

It’s likely a difference in the clock (oscillator) in your soundcards (or interfaces). Are one or both of you using a “regular cheap” soundcard or a “cheap” USB mic?

No clock is perfect and two clocks will never match perfectly and they will drift-apart. But good audio interfaces should be good-enough and they should stay in-sync for longer than the length of a song. Pros use super-accurate master clocks (and interfaces that can use a master clock).

Musicians usually notice the related pitch-shift more than the time-tempo change.

It can be tricky sharing project files.

But it would be better to share WAV or FLAC. MP3 is lossy compression and although the quality loss may not be noticeable, “damage” accumulates with each generation of compression. If you want MP3, it should be compressed ONCE as the last step.

One of the side-effects is that MP3 adds a few milliseconds of silence to the beginning & end, but I don’t think that’s your problem.