As Matthew Gatland suggested in MC-191947, this was made into a separate issue.

The Bug:

An entity that is capable of being sent to the nether that ends its velocity based movement inside a nether portal cannot be sent to the nether unless it is ticked again. This prevents a capable entity from being sent to the nether in the same gametick that its movement ended.

This is noticeable when the entity's end position is in a nether portal in a non-entity processing chunk as it moved into it right before the tick ended, but after the nether portal tick happened and it will not go through the portal on its next gametick as it is now in a non-entity processing chunk and won't tick.

How to reproduce:

1. Create a flat world and teleport to 1000, 10 1000 via /tp 1000 10 1000
2. Build a nether portal @ the cords 1009 4 1000 to 1009 4 997
3. Light the portal and go thru to load and link the other side.
4. Build a wall behind the portal on the east side.
5. Wait 15 seconds so that the portal no longer loads the chunk as you came through it.
6. Turn your render distance down to 2 and your entity distance up to max
7. Go to the coords 989 5 1000 to make the portal chunk non-entity processing
8. /summon pig 1007.00 5.67 999.45 {Motion:[4.0,0.0,0.0]}
9. Observe that the pig is still there.
10. Video for the steps: https://youtu.be/2dg7jUIGeho

Code analysis:

A code analysis was done in a previous issue closely related to this one. I will link the old one as the code is more or less the same with one variable charge that is mentioned at the top. 

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

Solution: 

A solution is simple: move the nether portal ticking after the move method.