Every time I used the Nether portal, in both the Overworld and the Nether, what happened was I left the Nether portal and then, about a second later, I was pushed back to inside the Nether portal. This happened around 10 times before it let me leave the portal.
Environment
Java edition on Windows 10 computer. Occurred on 1.13 survival world although I tested it a new creative world and the same issue occurred.
Just sounds like general lag based rubber banding. If you've had your world open for a while, restart your game and it should fix it, unless you're on a server.