I'm playing in Minecraft SnapShot 1.12 pre 2 and the /execute commands detect attribute will not work.

In one example. I'm trying to give Zombies a wither effect in they stand over coarse dirt here is the command:
/execute @e[type=zombie] ~ ~ ~ detect ~ ~-1 ~ dirt 1 /effect @e[type=zombie,r=0] wither 10 15 true

I eventually simply changed it to:
/execute @e[type=zombie] ~ ~ ~ detect ~ ~-1 ~ dirt -1 /effect @e[type=zombie,r=1] wither 10 15 true

Which still should work. But it didn't.

I've also gone on some of my other worlds that have working /execute-detect commands and none of those commands worked anymore either.