I think this has to do with the server processing the spell cast.
Example:
1) T = 0.00ms ; Player A cast slash ; Slash is queued for execution.
2) T = 0.50ms ; Player B cast seal ; Seal is queued for execution.
3) Server execute all spells queued between T = 0.00ms & T = 0.99ms
4) T = 1.00ms ; Player A slash and Player B seal concurrently.
1 way to fix this is reduce the interval for server to execute those spells.
For example, Player A slash is executed at T= 0.40ms, then Player B will be dead and unable to seal target at T= 0.50ms.
By doing so, I am afraid there will be more "bugged" skill.
Based on my observation, a spell is "queued" when you click spell on target.
Whenever a spell is "queued", if the server unable to execute the spell, then the cooldown will start ticking regardless of the execution.
For example, Cruz A cast slash, but he is slowed by cripple, Cruz A unable to reach target to cast slash within 1 second, Server execute all spell queued, the slash is bugged and Cruz A slash is under cooldown.
If my observation is correct, by reducing the interval for server to execute spells, a lot of skills will be bugged.
I could be wrong, just guessing.
Edit:
Again, if my guess is correct, by adding micro stun may not fix the root of the problem.