Aegis of Zeonia glitch

When fighting a Rek'Sai I attempted to stun to prevent her escape, at the same time she tried to tunnel. My ability went on cool down preventing me from following up, also she was not stunned the least bit. The effects I would expect are either she stops mid animation(she hadn't hit her dive before I had completed Aegis of Zeonia) and stuns, or when she surfaces she is stunned as my effect lasts longer than her ability animation. The case I witnessed was that she carried on unimpeded. Is this by design? It seems rather pointless that she can essentially shake all cc if she is hit while attempting to tunnel. Side note, I'm not sure of the priority but Pantheon's Aegis of Zeonia stun is canceled out by almost every other champions stun or cc. Another instance that happens repeatedly is Renekton's Ruthless Predator, which is melee, takes priority of a flying gap closing stun that is in range not only long before his melee ability but also has to hit before an auto attack animation, since his move is already being carried out. Whether it is an issue with Pantheon himself who I play quite a lot of or these champions specifically or individually I don't know. Looking for some feedback.