Musicxml export does not included altered ontime values
Related Bug: #288687: Musicxml export does not include notehead specification when using custom notehead definitions
Similar to the notehead issue, where notehead shapes are not specified, exporting to musicxml also loses any "ontime" changes to notes. I believe the musicxml element missing is the "attack" attribute:
"The attack and release attributes are used to alter the starting and stopping time of the note from when it would otherwise occur based on the flow of durations - information that is specific to a performance. They are expressed in terms of divisions, either positive or negative. A note that starts a tie should not have a release attribute, and a note that stops a tie should not have an attack attribute. (https://usermanuals.musicxml.com/MusicXML/Content/AT-MusicXML-attack.htm)
To reproduce the bug, export score with altered ontimes to musicxml and reimport. It appears that the Musicxml support in Musescore 3 was never fully completed. Unlike the notehead issue, this issue is not a regression as Musescore 2 also did not support ontime changes when exporting to Musicxml.
My system: OS: Windows 10 (10.0), Arch.: x86_64, MuseScore version (64-bit): 188.8.131.5215, revision: 526f80e