Cassiopeia Miasma Freezing Minions

When using Cass' new W I noticed that sometimes minions standing in the poison would just stop moving completely. There didn't seem to be any particular pattern to how or why they would stop, and they weren't being terrain or creep blocked. I don't know if this is some kind of weird side effect of the new grounding mechanic but I thought I would bring it up. Has anyone else experienced this?