In the past I talked about how secret levels like gargoyles' lair don't remember the amount of coins and gems you had when you entered them.
Now I found more issues with these kinds of levels which may or may not be related to that.

If you enter the secret level with a bird and then lose said bird while in the secret level, you'll get the bird back when you return the original level, as if you never got hit or went to that level to begin with.
The same happens in reverse: acquire a bird in a secret level and hold on to it until the end but the moment you come back to the main level the game acts as if you never had a bird in the first place.
Shields also have this problem, you completely lose whatever shield you had when you finished the level regardless of how much time you had left on the clock. If you enter the secret level with a shield you will carry it over like with a standard level but upon your return the game will suddenly "give" it back to you with the timer at the exact point it was when you entered.
Similarly, no matter how many fastfires you collect in the secret level your rate of fire will reset to what it was beforehand when you return.


Oddly enough weapon powerups and ammo count aren't affected by all of this and carry over properly from secret to main level.