if an anvil has nothing underneath and isnt falling, it gives a particle like sand/gravel would
How to reproduce:
in repeating command blocks, run these two commands:
setblock ~ ~10 ~ minecraft:anvil
execute as @e[type=falling_block] at @s run data modify entity @s Time set value 0
then simply watch the anvil an you will start to eventually see particles
That's intended functionality, any gravity-affected block will spawn particles beneath it, regardless of if it's "powder" (such as sand, red sand, gravel, concrete powder)