Thresh bug with Face of the Mountain and build items

I have always had this problem ONLY with Thresh where when I buy Relic Shield, Targons, or Face of the Mountain my execute doesnt alasy work on minions properly. The other day I was playing a ranked game as Thresh and kept complaining about it out loud to myself when I see an in game message from a buddy of mine spectating my game and he says "damn i hate how that thresh execute doesn't always work" and then I when i knew it wasn't just me I was thinking about watching my replay just to make sure its not a crazy coincidence that me and my buddy both have the problem and then literally the next game I played there was a different guy playing thresh on my team and bitched about the same problem. I think its safe to assume that this is something that needs a look at.