After Goku attempts flying and fails Whis states the arena is controlled so abilities like Flight cannot be used. Given Grand Priest made the stage is fair to assume he made it that way.

The Grand priest probably had his stage built in a way that all ki users were nullified of their capacity to take flight.

So all participants at the Tournament of Power were not allowed to fly, but after Jiren’s battle with Hit, it was shown that Jiren, while meditating, can float above the ground.

Possible Explanation’s

To be fair, some of the fighters like Jiren and even Ultra Instincts Goku are fighting at the levels of a god of destruction. The world of the void is infinity, and Jiren’s mere presence caused infinity to shake, and even when Goku emerged during his Ultra Instincts technique, it was stated that the world of Void is shaking. I think when their power is as enormous to ripple across infinity, such rules won’t apply to them.

Grand Priest

It’s more probable that jiren were levitating without ki since he was “Floating” only a couple of meters above that rock.

Maybe levitation is still probable, but you just can’t navigate in mid-air. Goku was levitating during the Spirit Bomb struggle with Jiren, and he didn’t even have UI yet.
I expected the immense energy from the spirit bomb was keeping Goku in the air, not that he was flying.

We see Android 17 use his energy to create a form of footing, and he stands on that to eliminate Kakunsa. The special force or specific gravity responsible for stopping flight should have prevented that layer of footing from existing, but it didn’t. So in other words, Android 17 could’ve sat on that for as long as he wanted and it would appear even as though he was floating. Jiren’s energy is apparently a lot superior to android 17 so whatever special forces of gravity responsible for preventing flight aren’t strong enough to hold Jiren in place.

I think it’s just the level of strength Jiren is putting out is higher to the amount of power the Grand Priest used to cancel the participant abilities, like how in the Universe 6 arc fighters were able to break the barriers made by Whis and Vados, but I wanted to hear what others thought.