I've had this bug pop up from time to time over the years. I found something that clears it up for a while but it always eventually comes back. It pops up pretty rarely thankfully. Talk to the sim that is locked in the "top left", number #1 position in the relationship panel. I found this easiest to do by just having your sim at home and then invite them over. When the #1 locked sim shows at your house, talk to them. For me, this bumps the locked character out of the #1 position to about the middle of the relationship panel. The sim that was to the right of the #1 position should now be the new #1. Repeat the call thing with each sim. Eventually you'll notice the initial locked #1 sim when you started the process working their way back to the #1 spot. Either right before they get back into the #1 spot or are at the #1 spot again, the relationship panel goes back to normal somehow. It's been a while since I've had this bug pop up so I forgot exactly when the Relationship Panel rights itself again. The Relationship panel is now "unlocked". It goes back to who ever you are currently talking to being in the #1 spot.