This is not a new issue, I started finding it in previous versions.
Step 1: Export mix as MP3
Step 2: Export a different, SHORTER mix, with the same file name. [File Already exists!] Overwrite previous version.
Check the file - it will be the new (Step 2) mix, but a portion of the old Step 1 mix will be tacked on the end.
So Overwriting the previous version doesn’t erase the old file, if the old file was longer, the last bit of it is added to the end of the new file. This issue only seems to affect exporting as MP3.
The issue seems 100% repeatable, I work around it by deleting the old file in Finder before exporting the new one.