The nether portal disappears when you walk through it, trapping you in the nether with no way out, and the nether resets when you aren’t in the nether 

Edit: Seems to only occur when playing multiplayer. 

To recreate: 

1. Have at least 2 people travel through the portal, leaving one or more behind
2. The portal will not be their upon player entering nether, Players in the overworld can still throw items through the portal