Nether Portals Not Transporting Player

As per the title and the attached image, the nether portals are not transporting the player, making the game virtually unplayable.

Entities can enter and be transported with no issue, whether that be mobs or dropped items. Currently , my only solve is to drop all items into the portal and kill myself (losing all exp. In the process) and picking up my items on respawn.

This is a temporary fix as after using the portals a few times they stop working again. I cannot recreate the issue at will so I cannot provide any more info. Use over time seems to be a deciding factor.

Thanks in advance,