Alright so I was having fun with Elytra and fireworks underwater (by falling into the ocean with the elytra on and speeding myself with the firework rockets) but then I've started walking on the ground and noticed I was walking and jumping faster than normal but the other mobs were still having a normal speed
I've decided to record it to show some possible proof. Also, as I said at the end of the video, I decided to try /kill and it actually fixed the bug. I don't know how to do it again but it happened.
[Link to the video (YouTube)|https://youtu.be/qEaE322H7S0]
As Urielsalis said in MC-134819, this was confirmed in the release notes, will be fixed in the next snapshot.