No copy and paste in Musecore 4.2.0.233521125
On the grand staff for a new piece in B-flat, select a region, Ctrl+C, Ctrl+V ... and ... nothing. Do it from the menu: same nothing. Same for just one clef, one note, C-major. Ctrl-N, "c" , Ctrl+A, click, Ctrl+V - nothing.
OS: Ubuntu 23.10, Arch.: x86_64, MuseScore version (64-bit): 4.2.0-233521125, revision: eb8d33c
Comments
Works fine here as always (Windows10, MuseScore version (64-bit): 4.2.0-233521124, revision: eb8d33c)
Maybe verify in the Selection filter (menu View) : shouldn't you have unchecked Voice 1 (for example)? Check "All" again