I noticed while playing around with the Crown of the Primus that the stun from enemies entering or leaving the slow time area does not always apply to enemies, i presume its running as a “tick rate” style stun rather than a cross this line and get stunned. Its mostly with enemies that are moving fast or that you are moving towards. I am guessing they are making it through with just the perfect timing to nullify the effect, right in between ticks.