Separate Tracks not working

I watched the recent yt tutorial on separating tracks in the see music midi editor. I have to say it doesn't work for me. If I select the notes in the editor and click on the "2" bar, the editor erase all notes beside the selected ones and breaks the track. I have just downloaded the latest see music app for Mac using

MacOS 10.15.7. I cannot upload the midi file here, so I put a link to the download: http://www.piano-midi.de/midis/beethoven/mond_1_format0.mid