It’s because she’s never really felt like a solid character.
Exactly. As a result, I think Belle has become a placeholder for a series of facile, progressively more and more irritating stereotypes: Belle the maid with the heart of gold (cough cough, no pun); Belle the damsel in distress; Belle the “smart because can spew out random facts” hot librarian; and now Belle the mobster’s hysterical trophy wife. At each turn, the writers have failed to engage with, or try to subvert any of these tired tropes — instead, they’ve just moved onto the next, even worse one.
To add insult to injury, the plot meant to hold these together is pretty preposterous too.