"Delete All" does not delete all..

In my current project (created just for testing purposes), when in write-mode I “select all” (CMD-A) and hit “Delete” some music in the drum stave stays there…
I don’t think this is expected behavior…?
Project attached.
My Kind Of World.dorico.zip (1.12 MB)

The reason that drum material is left behind is that it cannot be selected by Ctrl+A because it is hidden behnd a slash region. You can’t select things that aren’t drawn, so until the covering slash region is removed, that music can’t be selected and deleted.

Hi Daniel, thank you for your quick reply.I understand. Furthermore I also noticed that in the drum stave several drum clefs appear after the notes that were revealed after the delete operation. I do not understand, how they got there. As far as I remember, I did not enter them explicitely. Do you have any explanation? Am I missing something? Thanks in advance!

Those aren’t clefs: they’re rests. Pairs of double whole rests, or more accurately “longa” rests.