Going from portal A in the nether, into portal B in the overworld created another portal C, despite the fact that portal A and B were already linked.

This appears to only happen when the portals were already generated and linked in a previous version.

The picture shows one of the portals in the overworld, that I would frequently use to travel from one place to another using the Nether. After downloading the 19w36a and coming back from the Nether into the overworld, a portal generated next to the one that was already there. 

This can be easily reproduced by:

1) Creating a map

2) Linking two portals

3) Log off, and update the world to 19w36a

4) Cross the portal (It doesn't matter whether you come from the nether or the overworld).

5) You should be teleported to another portal than the one originally created.