The bug
It appears that the structure block saves its mode in both the block state and its nbt data.
Expected: it is only saved in the block's states or nbt data, not both.
How to reproduce
Place a structure block
/setblock ~ ~ ~2 structure_block
Enable the debug menu and hover over the block and see the mode block state
Use the
/data get
command to to read the NBT/data get block ~ ~ ~2 mode
Still in 18w50a.