STATUS: This issue has been solved.
A new thread has gathered more attention than this one, so in order to help noticing and solving the issue, all new feed back should be posted here: http://boards.pbe.leagueoflegends.com/en/c/champions-gameplay-feedback/zLPO0yiL-big-lissandra-bug-on-pbe
I was testing Lissandra's E in the updated Summoner's Rift walls when I noticed I was not freezing enemies in place with her W (not even at max level), so I looked into the problem and came up with some details:
-Champions were dealt W's damage
-~~Champions would stop their current animation abruptly, and then proceed to resume their previous activities immediately after, resulting in them stopping for about 0.25 seconds and then keep walking as if nothing had happened~~. EDIT: It seems like they stopping momentarily was just a reaction of the bots after recieving W's damage. They normally wont stop at all.
-Champions didn't present the root particles at their feet like they should do.
-Champions didnt have any tenacity items.
-W's root were applied both visually and practically on minions the way that it should be.
-I was using Blade Queen Lissandra skin (apparently happens with all skins)
-This was on an intermediate bots 5v5 custom game in the updated Summoner's Rift. I have confirmed that it happens in other maps too.
-Thanks to MagicHour we've managed to confirm it happens in queue matches (PvP) as well.