Beams get lost when some notes are moved across bottom stave to top stave with Command+Shift+Up Arrow.
Enter 8 x 1/8 notes into a bar of bottom stave for piano. They will be beamed up together as 4 x 1/8 and 4 x 1/8. Flip the last 4 notes to the upper stave by Command+Shift+Up-Arrow. Then the beam will be lost, as it renders them as 4 x 1/4 notes. However, they still play as 1/8 notes. If you enter the 8 x 1/8 notes into the upper stave and flip down the last 4 notes to the lower stave, it works, i.e., they are correctly rendered as 4 x 1/8 notes with a beam connecting them together.