Unison collisions fail in MS-created MP3's
OS: macOS 10.15, Arch.: x86_64, MuseScore version (64-bit): 18.104.22.16837, revision: 148e43f
For the enclosed (simplest test case) score, "Export to MP3". The whole-note is cut off (clipped) by the quarter note, a "unison collision clipping". This problem was fixed for live playback from the score way back in MuseScore 2, but persists when MP3 output is produced, and, since the .com site works via MP3 production, all such scores posted to the site fail unless all unison collisions are located and hand-tweaked as they were before that fix (but that is now much more difficult because of the difference in behavior).
Apparently, some patches were posted and pulled/unpatched and maybe lost. @mirabilos (who was responsible for the patches) describes his understanding of the chronology on the discussion linked above.