You can rationally know that your partner is faithful and still feel a sting when someone openly flirts with them. You can respect their freedom, demand no restrictions at all, and still briefly feel afraid of being replaced.
Because jealousy contains more than suspicion of deception. It can involve comparison with another person, fear of losing attention, anxiety about an important bond or a reaction to a change in familiar closeness.
So the fact that jealousy appears does not tell us very much about the quality of a relationship on its own.
What tells us considerably more is what happens after it appears.
There is a big difference between saying, “I suddenly feel jealous and I want to talk about it,” and demanding a phone password, banning friendships or deciding which people your partner is allowed to see. An emotion can arise without our permission, but our behaviour still requires our participation.