Before the Bug Occurred (and any info that might relate to the bug)

I was playing Minecraft with my friend, it was just me and him and I was hosting the server. The world is pretty laggy if that helps. There's a ruined nether portal that we fixed and used that is not too far away from our main portal. It is linked to the main portal as when we go into the ruined portal, it usually takes us to the same place our main portal takes us. But since our main portal was made first, no matter what portal you go through in the overworld that takes you to the same place in the nether, when you go back it will take you into our main portal. Now, there is an xp farm right next to the ruined portal so when we are done in the xp farm (eg. mending our items or getting more xp), we go through the ruined portal to get back home. But today, that was not the case. My friend was at the xp farm and when he was done, he decided to go through the ruined portal.

When the Bug Happened and After it Happened

My friend went through the ruined portal but he didn't get sent to our main nether portal. He was sent to the same coordinates that the ruined portal was at (he was supposed to be sent around 415 x, -310 z, but he was sent to around 4000 x, -2500 z!) There was also no portal where he spawned at all, so he went halfway back but then I had to come then because he ran out of food. Thankfully we made it back, but we haven't gone back inside the ruined portal since. (also I can't send any photos because my friend didn't take a screenshot and I didn't experience the bug)