I have a structure spawning upon world creation using World Primer. The structure also has a a few structure blocks built into it to spawn other structures when activated by the player. For some reason World Primer refuses to place the structure blocks, despite them being listed in the nbt file. The spots in the structure where the structure blocks would be is just air.

As we spoke about in discord, the blocks show up fine when exported as a schematic, but I will leave this open at your discretion, since they still don't show up when exported as a vanilla structure nbt.