Nether Portal Creation Coordinate Calculation Error

I had existing nether portal at [153,111,-84] and corresponding overworld portal at [1230,81,-669]. These are a perfect match on X and Z, with the nether portal lying 30 blocks straight up.

I then removed a crucial obsidian block from the overworld portal (thereby deactivating it), in an attempt to move it the pair to a location I preferred, a bit to the southwest. The old nether portal was unharmed.

I built my new nether portal at [141,120,-79], which I expected to spawn a new overworld portal around [1141, 111, -623], on a flat open space, as there were no active portals nearby in the overworld.

Instead, a new overworld portal was created right next to the destroyed one at [1230,81,-669], even creating new dirt (the old one was near the edge of a cliff). I removed an obsidian block from this new portal and tried again, from the new portal in the nether [141,120,-79]. Again, it created a new portal ~100 blocks away from my expectation, right next to the deactivated overworld portals corresponding to the still-active nether portal. I repeated this 2 more times, so I now have a cluster of 5 deactivated overworld portals.

Lastly, I removed an obsidian to deactivate the old nether-side portal at [153,111,-84]. Once this was gone, entering the new nether portal at [141,120,-79] spawned a new overworld portal as I expected at [1131,119,-632].

Therefore, I believe that the nether portal spawning algorithm substituted the coordinates of the existing nearby active nether-side portal for those of the nether-side portal being entered when calculating the target destination for the overworld portal. This doesn’t seem to be intended behavior.