That is really curious ... did it happen to you too or did you saw it only on stream? Did the streamer react to it - commented on it?
I am asking, because I have suspicion that it is video compression artifact. Item stack count numbers have black outline and it seems to me video codec made it barely visible at first and then very pronounced as inventory resorted and screen changed a lot in single frame.
Thanks for the report.
Good news is that it is fixed in 0.17. Bad news is that I don't know what was causing it, and should find out, but now is not good time to work on this.