19w42a, 1.15 Pre-release 1
Window 10, Java 1.8.0_51 64 bit
I created two portals in a single player 14.4 world (seed 7740105727846060159) and used them many times successfully. (The first one was automatically generated, the second one I created manually.) After loading the world in this snapshot (19w42a) the first portal I created, when travelling back from the Nether, created a new portal 6 blocks to the north. Going to the Nether through either portal takes me to the original portal in the Nether, but coming back always takes me to the newly created one. After disabling the new portal by removing a block, going through the original portal and coming back again created another new portal in a different location. I tried multiple times and every time I get a new Overworld portal created in a random location. Disabling the portal with water and leaving the frame whole had the same effect.
Original Overworld portal: -132, 72, 75 links to Nether portal at -16, 81, 9
New Overworld portals created at:
1) -133, 72, 69
2) -144, 72, 73
3) -128, 71, 62
4) -121, 74, 62
5) -144, 72, 61
6) -133, 71, 60
7) -137,63, 84
The other portal at -420, 33, 34 linking to -53, 83, 8 works correctly.
2019-10-23-20.48.15.png shows the original portal and the first new one created.
2019-10-23-20.47.48.png shows several of the new portals created.