If you use the particle command to generate falling_dust, block, or item particles but haven't typed the block or item, it says "unknown block 'minecraft:'" or "unknown item 'minecraft:'" without suggesting blocks or items to use, like in the fill, setblock, or give commands. It also does not suggest block states when you do type a block, for example: /setblock ~ ~ ~ oak_slab[ suggests the "type" and "waterlogged" block states, but when you do /particle block oak_slab[ it only gives the error "Expected closing ] for block state properties", where it should also suggest "type" and "waterlogged".

This isn't a super major problem as the command is still useable, but it makes it harder to use for those 3 specific particles, since you can't autocomplete typing.