This is the double levitate bug.
If you get a 2nd effect with levitate that stacks with (or replaces) your other levitate buff. When one ends (or the original gets replaced) it makes the levitate effect no longer work.
It's a classic bug.
It was very prevalent during PoP when Druids/Rangers would have Spirit of Eagle and then a higher Druid would cast Flight of Eagles on them. It would kill the levitate. At which point it was bugged and fixed.
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