My sim randomly lost her eternally faithful reputation

Reputations are world-specific. What happens in Champs Les Sims stays in Champs Les Sims, or something like that. A sim can be eternally faithful in one world and a downright dirtbag (or whatever the most negative one might be) in another world. They may even have different partners/spouses, one per world, as long as worlds do not collide as the expression would go -- the partners cannot/should not ever be in the same world at the same time.

When return travel happens, sometimes the metric has to begin again to give credit by way of the moodlets.

Reputations are also how other sims see the subject sim in question and are what they talk, whisper, gossip about if they have nothing better to do. Positive or negative, they aren't necessarily based in reality or on logic. I once had a sim with one partner but 27 Romantic Interests in all different worlds, including some back home, pick up the Faithful moodlets when back in his homeworld where his partner was. I thought that was pretty funny, so I let him keep it until they finally broke up. :)

2 days after they got home, I got another pop up saying Connor has an eternally faithful reputation and now they both have the moodlets back. I think it’s just bugged.