She forgets how to be humble is present tence and implies a momentary lapse. There is hope that she may rember in the future
She has forgotten how to be humble is past temse and is final implying that she will never agin be so
forgotten is past tense, while forgets is present continuing
Certainly not when you demand with exclamation! Ever heard of manners?
The meanings are very similar but actually "she has forgotten" is most correct. "forgets" implies something that is happening now and continuing to happen. Or if you wish to imply that it keeps happening, a correct way would be "She keeps forgetting to be humble." or "She continues to forget humility."