Why does XML import of single parts yield better results in MuseScore and Finale than in Dorico

See this thread:

There are some workarounds mentioned in that thread that you can use, but it is worth restating here that just because the other programs import the multimeasure rest correctly doesn’t necessarily mean that it is correct MusicXML. Errors can result in different scenarios - some programs may accept invalid syntax while others do not.

Also since it sounds like you have Sibelius 7 yourself, you might want to install Dolet for Sibelius and use that to export instead (via the plugins menu) as it may give different results - one of the versions supported Sibelius 7, I know I had to use that old version before I upgraded. That wasn’t suggested in that thread because that person didn’t actually have Sibelius, they were sent the XML by someone else.