There isn't a known issue with Ailil and Brimstone, so I'm inclined to say this is just bad variance

I mean, that's too much

I've tried more times (100 hits) and got 0 of them

Isn't 555 ACC not enough maybe?

let's run down a few questions:

Are you sure you have the Brimstone blessing on the right champion?

Are you sure the Sand Devil is asleep? 

Are you positive you have accuracy?

I feel like this has to be an Occam's Razor situation where it's a simple explanation, the other route is record a video of some sort to document and send into support 

I've tested it with other Champs

So what I've found - stage 19 has this reproduced for Richtoff (other stages are fine)

With 525 ACC it just failed to land any Brims there, while stage 21, 22, etc. are working properly on his A2

Poisons were laning fine, so ACC works there and it's this specific stage

Hi there, 

I haven't heard anything back/found any other instances of this yet but one question: why accuracy/Brimstone on an Ailil? 

And I just tested myself and landed Brimstone just fine on 19 so I'm a bit curious what's going on on your end.