The bug

In the linked video, I showed that an entity that gets moved into a portal does not get sent to the nether, even if it ends up in a portal at the end of its movement. The entity only gets sent to the nether if it ends up in a portal on its next gametick.

I summoned a pig with 

/summon pig 1002.39 5.44 962.00 {Motion:[0.0,0.0,-3.5]}

and had a portal one block from the edge of a chunk border. The pig movement ended with it in a portal, but did not get sent to the nether.

Any entity that is in a nether portal even for one gametick should be sent to the nether.

A fix for this should be to tick the portal after the entities movement has been processed. 

Video

https://www.youtube.com/watch?v=oK1fkszTrZA

Code analysis

https://bugs.mojang.com/browse/MC-191947?focusedCommentId=754320&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-754320