Concrete can be shown as identifiers such as colored_concrete, while planks still use "wood_type" in the commands, which doesn't work. This seems to go the same for wood, stained_hardened_clay, and glass. Likely other blocks are affected, but it seems like the list is a mix of identifiers and block states.

Example

/fill a b c x y z concrete

This command will show the various colors of concrete.
 

/fill a b c x y z

This command will not show the various wood types of planks.

Note

I am unsure if this is a duplicate issue or an issue at all that requires another way of doing it, but please let me know if this isn't an issue!