There is a bug with "/execute if" when you nest multiple arguments.
For example, if you use a command like "/execute as @e[type=creeper] at @s if block ~ ~-1 ~ concrete run effect give @s levitation" you would normally expect that creepers that are standing on concrete will get the levitation effect and creepers that are not standing on concrete won't. Because of this bug even if only one creeper is not standing on concrete the execute fails for every single executing entity (creeper).
I'm guessing that happens because the game proceses all selectors in an execute command first and determines if the execute was succesful later, instead of checking for each entity/selector individually.