Are the modifiers of Shields of Cthulhu set per night?

Are the modifiers of Shields of Cthulhu set per night?

I fought 3 Eyes of Cthulhu in one night, and all of their Treasure Bags got me Warding Shields of Cthulhu.

I'm Softcore in a Expert Large world on PC.

The loot bags in Terraria are totally random, and its contents are chosen by an RNG (Random Number Generator)

In your case, you just got lucky. Loot bags contents depend on the boss it comes from and is then chosen with an RNG, along with any modifiers the loot may have, and is not dependent or chosen with the cycles of night and day... which is rather unfortunate... because it would make it so easy to get good loot.

But, I suppose that's exactly why that's not the case.

I should also mention that being on Normal or Expert mode does not affect the chances of getting loot, nor does it affect what kind of modifiers will be placed upon the loot, Normal mode having the same generation, but only without the bag to hold all the loot, the loot being immediately generated upon the death of the boss, while Expert mode itself only adds extra loot to the generation table, I.E. The Shield of Cthulhu that is available in Expert mode, but not Normal mode.