When you return from the nether, it makes another portal and you spawn somewhere else

I first discovered this on one of my worlds. Normally when you return from the nether it spawns you at a portal you made (If you have more than one portal, it normally it spawns you at the original one) At first I thought it was my world being weird, but on my uncle's realm when I came back to the Overworld for food, it made another portal and I didn't spawn at the portal my friend made. That's when I realized that it was Minecraft being weird. Please fix this.