When you see a youtu.be link that means its the previe video immediately after upload (i.e. still processing before it makes the link official per say).
As you can see something was clearly inserting "3D" into the code, I believe it tried to resolve the youtu.be link but something went wrong. I would check to ensure this does not happen again because normally things just don't go wrong out of the blue - you know? .
I was using the other link yesterday. The full one.
I think what happened is the video got highlighted in editor and then linked.. That was why I was wondering if it was happening to all videos or just the one. lol Something new every day bro!