During a co-op save my friend (who is playing an Oath of the Ancients Paladin) broke his oath after killing a defenseless/knocked out Goblin. He restored his Oath successfully and all was fine.
Fast forward to today (and about 4+ hours more into the game after the previous Oath), his Oath seemed to randomly break again. We didn't have a save too close to this happening and we couldn't figure out WHY it broke, but he had the extra gold to pay. We did a long rest to get to camp, he paid the 2000 and chose to restore it. After paying, he did not get his Oath restored. We loaded our quick save (I quicksave entering camp) and again, he paid the gold but no oath was restored. We completed the long rest and wanted to try again, but the Oathbreaker Paladin was no longer in camp at all.