Help version 1.12 commands no longer work the same as in 1.14 or more.

Hello everyone, I tell you my situation. I already advance in advance that I don't fully understand the mechanics of the commands in minecraft but I have certain knowledge.

Before in version 1.12 I used this command on my map:

"/execute @a ~ ~ ~ detect ~ ~-1.8 ~ soul_sand 0 /effect @a wither 6"

With it, I detected a player (or entity if I changed @a by @e) if it was located on top of a specific block and another command was executed, in this case /effect. But now i'm using minecraft version 1.14.1 .

The problem is that with the update at 1.14 the command blocks already say that it is incorrect. I have tried several ways until this command worked at 1.14.1 :
"/execute if entity @a if block ~ ~-1.8 ~ minecraft:soul_sand run effect give @a wither 6"

My problem is that this command only works correctly if I execute it myself, and not if I put it in a command block as before, I have tried more combinations but nothing. Someone would know how to make this command work, in my case I would prefer not to put Scoreboards to make this work, if this is possible, of course, since I would like it to apply to everyone without having a specific characteristic (for example "Deaths, experience , achievements ") to work with any player or entity.

Anyway, I'm waiting for your suggestions on how to make this possible...

Thanks to everyone.