Creating other nether portals on top of trees in 1.17

So, I have a nether portal with the corners filled in with pumpkins because I didn't have enough obsidian, and when I go through it I spawn in a cave in the nether with no discernable way out. That's not too big of an issue though, when I go back through the portal however, I come back in a new portal on top of a tree. The first time this happened, I just broke the other portal and used my own, but when I came back the second time, the game created a new portal a few blocks from where the one I broke was. Please help!