But it isn't just an "end zone rule".
https://profootballtalk.nbcsports.com/2015/11/15/reversal-of-beckham-touchdown-was-the-right-call/
"The league’s position, per a source with knowledge of it, is that the call was absolutely right. A catch is complete once the receiver gets two feet down and maintains control long enough to become a runner. There’s a time element, and Beckham failed to meet it because the ball was knocked from his possession as soon as his second foot came down.
When the ball is knocked loose at the same time as the second foot hits, the pass is incomplete. Period. If he has it long enough to get a third foot down or to turn or to brace for contact or to spike the football or to hand it to the official, the pass is complete.
The requirement applies wherever a catch occurs, in the end zone or in the field of play. And while the ruling on the field complicates the situation, the decision that indisputable visual evidence existed to overturn the ruling was the correct one. The defensive back knocked the ball out of Beckham’s possession before he held it long enough, and the easy solution is that he should have held it a little bit longer."
Key line: When the ball is knocked loose at the same time as the second foot hits, the pass is incomplete. Period.
I think that applies anywhere, not just the end zone.....
On top of which, several commentators said he was starting to celebrate to Butler, instead of hugging that ball....