Its actually quite simple.
Each spin is (and must be) independent to all previous spins.
The Jurassic Park one is an easy answer, and it also works for Wish Master and other games with persistence...
The chance of triggering the "feature" (I.e T-Rex mode) must be constant on the press of the start button. Once the "T-Rex" mode is triggered, all the spins in that mode must be independent but because the T-Rex mode is active, it is probably picking from a different set of reel bands.
The reason it is allowed is because the feature starting was still randomly determined, and the spins in the feature are still randomly determined. The important thing to remember here as well us it's not hiding it from the player... it is clear the game mode has changed and therefore it is fair to assume the player would expect different reels to be used.
Regulators dont like (and dont allow) it where things like this are done and hidden.
It is a different set. Basically, while the T-Rex Alert "featurette" is active (almost always for exactly 6 spins, but in the odd rare case, 12 spins),
there are 35 wilds added to the reels.
I am presuming that this is to replace all of the scatters that would normally be present within the standard base game reel set,
as well as chucking in a few extra wilds in isolated spots.
I am presuming this is because....
1. I've never seen a scatter land while T-Rex Alert is active.
2. I've seen a wild land
in isolation on the middle row during T-Rex Alert, but have never seen it happen in the base game.
Also, while T-Rex Alert is active, you cannot
increase your bet size, as there is a small padlock beside the word "bet".
Again, I am presuming that this to prevent the player trying to exploit the game by hiking his/her bet while it temporarily has "enhanced reels" active.
Because while the chance of landing a full screen of wilds remains exactly the same (as the stacked wilds remain 3 rows high),
the chances of a getting a wildline do increase, as there are likely to be 7 extra wilds per reel (assuming the 35 extra wilds are distributed evenly).
But all of this is clear for all players to see. So I don't see an issue here.