Nether portal brings me to abandond mineshaft underground instead of the portal in my base

What I expected to happen was that when i walked in to the portal to get back from the nether it will spawn me in my base (where the portal was)

What actually happened was it spawned me in the middle of an abandond mineshaft in a totaly different portal that never exsisted

Steps to Reproduce:
1. walk into nether portal
2. go back into the same portal;
3. walk out in to an abandond mineshaft underground

*the nether portal in my base is the only one that exsisted in my singleplayer world