Attacking through stuns [GAMEPLAY]

Like the tittle says, i've been experiencing champs that are able to basic attack (once) or cast a skill right after a stun, not at the same moment they get stunned or right before but it looks like it's when they get stunned, it is right after they get stunned. I've read several posts that have similar issues, like using hard CC on someone while they dash and getting attacked right after (once), or someone ignoring hard CC because of dash and/or attack modifiers. In my case the champ just gets stunned and is able to basic attack once (basic attack is in slow motion, but still deals damage) right after they get stunned without dashing or using attack modifiers. Also has happened a few times with some skills that are not meant to be casted through stuns like Mega-Gnar's W for example, that will not be interrupted if you hard CC him.

This happens to me usually with Irelia's or Renekton's stuns,but for example, with Brand's stun it has never happened. I dont know how to reproduce it because it just happens sometimes and i don't know why it happens.