Slicken's prone duration lasts 0 seconds when cast. Oddly enough, it only affected me today.

I have the same bug occur recently. It doesn't only effect Slicken for me though. It also effects Call to Slumber and Amplified Wave on my Cipher.

 

It doesn't matter if they hit, crit, or graze, it always lasts 0.0 seconds. The animation for falling prone will play then the enemy will instantly stand up.

It has already been fixed as of 1.04.

From what I can tell, this bug is still in place in 1.04. My monk's Force of Anguish is 0.0. He was using the Staelgar belt previously (prone reduction), but isn't any longer--other users have reported that any defensive prone reduction effects are affecting the party's offensive prone reduction. Hard to confirm, but my case matches what other people have reported, whereas my party members (e.g. Eder) who have never worn a Prone reducing item have perfectly fine knockdowns or prone effects.