Did something change with how a kill is determined?

I've just been noticing a lot of times when I've been playing in the past couple of months that kills don't seem to be attributed to the playing getting the last hit. I have noticed that I would get the last hit, but someone else would take the kill. At first, I thought maybe they just had a DOT effect going that I didn't notice, but I have also been given kill credit when I know for a fact I wasn't the last hit, and I didn't have any sort of effect going for delayed or bleeding damage. I'm not talking about kills by Pyke, I mean I somehow got a KS off of our Lux and later she did the same thing back to me when both events should have been otherwise impossible for us to "last hit".

And this really has only been in the past month or two. Six month ago, last year, two years ago, every last hit gave the proper champion their kill. Now feels rather different, like there is a damage percentage requirement within a really brief amount of time or something like that.