Incorrect font property of one text block

The source is the support request and related forum post https://musescore.org/en/node/270436.

The .mscz file is not uploaded correctly. Scenario to reproduce locally is:
1. Open the score
2. Export to MusicXML
3. Import this MusicXML
Result: error info mentioned here https://musescore.org/en/node/270436#comment-825370
Expected result: usual MusicXML import without errors

The reason is strange value for the font family in the one of the text blocks: https://musescore.org/en/node/270436#comment-825406.

Exported MusicXML file leaves the tag fontFamily with empty value. MuseScore shows font family as the first one in the list.