If you create a structure using structure blocks, entities can be saved with the structure. If you load the same structure, the entities will have the same entity data as expected.

If however you make the structure generate in the world with a datapack, some entity data is ignored.

I can confirm this happening with:

• "Size" for Magma cubes
• "HandItems" for Zombified Piglins, Skeletons, Wither Skeletons, Strays and Vindicators (NOTE: "ArmorItems" is saved correctly)
• "Passengers" for Minecarts

Does not affect "HandItems" for Piglins, interestingly.