Start off with all the notes on one stave. Then select the notes you want to move up a stave and hit N (for North), or select all the notes you want to move down a stave and hit M.
I tried that and it separated all of the beams.
Were they beamed together before you moved some of the notes to the other stave? If so, I’d hazard a guess you used Alt-M/N, not M/N, which is a different function. If not, you’ll need to beam them together either by altering the time signature or the Notation Options > Beam Grouping, or by right-clicking > Beaming > Beam Together.
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Ah yes, my mistake. I was using alt+M.