MuseScore 3 does not SaveAs correctly a new score when it contains a . in the name.
I will use my real example when creating a new hymn.
1. Create a new Hymn 595 file.
2. SaveAs Hymn 595.1 (the file created in my file folder is not a .mscz music file.
3. SaveAs Hymn 595-1 (the file created in my file folder is a proper .mscz music file.
4. Rename to Hymn 595.1 in the file folder.
5. Click on it to open it in MuseScore. (I can now edit it and save it properly; from this point, I can also Save As Hymn 595.2 or Hymn 595.3, etc., and the files created are proper .mscz music files.