Was the kill contribution timer changed?

Okay, so.. I've noticed in a lot of my games over the past few days that assists seem to be slightly less likely. Which brings me to - Was the time allowed for kill contribution reduced? It doesn't seem like anything drastically changed, but there's been several kills where I'll attack an enemy to get them low, then walk away and press B while my jungler dives in and kills them - This has always been a free assist for me, considering the opponent is dead before I even finish my recall animation. Lately that hasn't been the case - Quite a few times recently I've missed out on the assist on kills where I tagged them with enough damage to get them low then walked away.

Not a complaint or anything, just purely curious. It feels like kills that I used to get contribution on, have a lower chance of paying out for me these last few days. Were there any changes? Or am I just walking the timer line, and have gotten luckier in the past several years worth of games than I am lately?