The first portal I made to go to the Nether has been used for a while now. But there is a bug where if you reenter the portals in both dimensions in quick succession, it has a chance to ignore the original overworld portal and generate a new one within the 128 block radius. This is really frustrating seeing as I'm a busy player running up and down places. Discovered this bug when I changed my mind about something and wanted to head back through quickly