Multiple nether portals always teleport back to first location

I created Nether Portal A (NPA) at House A and have done extensive adventuring in the Nether from there.

After establishing a new location – House B --, I built a second nether portal — Nether Portal B (NPB)

After using NPB for the first time, I arrived in the Nether in the same location that I arrived from NPA (not sure if this is normal behavior). When I went back through the portal, I was delivered to NPA.

I later destroyed NPA, traveled back to House B and went through NPB to test it. My expectation was that, since I had destroyed NPA, it would return me to NPB when I exited the Nether.

Instead, the game generated Nether Portal C near House A (but not in the original location of NPA). On the upside, 18 free obsidian blocks.

The distance between House A and House B is quite considerable. The are on opposite sides of an ocean and require almost a full day in a boat to travel between them.