I made a structure with my mod. It had a mob spawner and a chest too. There was a mob in the spawner and some items in the chest.
I made the structure spawn in a biome and when I found it in runclient the chest was filled with items, there was the mob in the mob spawner and everything was working fine.
I exported my mod and tested it using a few other mods (that don't modify structure gen). I found the structure but both the mob spawner and the chest were empty.
I think that this is caused by mod conflict, I will later check the structure without using other mods.
(Sorry for bad English)
EDIT: It seems that no custom blocks, items in chests and mobs spawn, so this bug is not just for items in chests but for everything modded. It also happens when the mod that adds the structure is alone.