This is related to MC-113314, which was "resolved" as "working as intended". This is a follow-on ticket, showing how this "working as intended" grants players the ability to create infinite obsidian very quickly, which, in my view, is a serious exploit.

I created a nether portal in the overworld several hundred blocks from my base. I went to the nether through it and made a trek to my original nether base. Then I went back through the new nether portal in the nether, and ended up several hundred blocks from the portal I created earlier.

MC-113314 was the same root issue. However, what wasn't mentioned is what I did next. I thought, perhaps if I get rid of this new portal in the overworld, and go back to the one I created and repeat the journey to the nether and back again, I can get it to work properly.

So, I broke all the obsidian (collecting 14 obsidian in the process). Then I went back to the portal I originally created and went through again, and then back to the overworld.

Lo and behold, the game created yet another nether portal in the overworld. So, I did the whole process again. Sure enough, I could get another 14 obsidian.

So, this process allows a player to generate as much obsidian as desired, with no downside other than the time to break the blocks.

Some notes:

The new overworld portal that generated generated in a different place each time.

The first two times a new portal was created, it was created at the tops of very tall trees. This is probably related to the bug in MC-242348.

The seed is 177. The overworld portal I created was at -933/80/631 (just outside the mansion).

The corresponding nether portal created by my traveling through my overworld portal was created at -100/85/87. This isn't at one-eighth of the coordinates of my overworld portal, as it should have been.

I didn't record the locations of the first two portals created by the game when I came back to the overworld. The third one was created at -790/79/689.

I let the third portal remain, and after that I was able to port back and forth between that portal and the one in the nether. I was also still able to travel to that nether portal from the original overworld portal.