Please clarify what you mean by ‘bouncing around’, but you have essentially discovered a
bug of vanilla Minecraft. Nether portals are not magically linked; the blocks themselves hold no data. Instead, when you enter a portal, the game searches for the nearest applicable portal in the other world, and if it finds none, it creates it. If you have two portals close enough together (which is extremely close in the Nether and not close at all in the Overworld, as distance in the Nether is 1:8 with distance in the Overworld), then they will both lead to the same portal on the other side. And that portal will only lead to the closest analog in the Overworld, producing this behavior. Due to how distance calculations work with that 1:8 ratio, sometimes you can build a Nether portal, go through it to create one in the Nether, and have it create another new one when coming back to the Overworld!
Anyway, vanilla behavior. Most people have experienced this before. Nothing special or Sponge-specific.