On a failed save, the target obeys the Flee command by
[Spending] its turn moving away from you by the fastest available means.
- Does that mean they take the Dash action?
- If not, do they take the Disengage action?
- In case the answer to 2 is no, do they provoke Opportunity Attack?
I'm leaning towards 'yes' to 1 ('fleeing' seems to suggest 'dashing'); no to 2 ('fleeing' doesn't suggest 'careful retreat'); and 'yes' to the 3rd (since it's not 'forced' movement, at least as per PHB page 195).