I have also experienced this sometimes, but never experimented to see the cause.
To me it always felt like the system does not fully reset once it lands. In other words: when you start, it knows it is starting at zero, however when you go up and down, it has some offset, even if minimal, that can cause this to manifest itself, especially if not going full throttle. If I am not mistaken, I remember once starting with altitude not being zero and thinking it was strange.
But going full throttle on takeoff is not a solution. It is a workaround. If DJI is looking at this, I would suggest (if it is not done) to perform a full reset of parameters after idle command.
Also worth noting that I have noticed a few times that landing and holding the throttle down, did not idle the motors, and as a matter of fact, after holding for a few seconds, they revved up. I usually CSC after this happens.
None of these are too critical, but all this information can help make the Phantom a better device.