Lisa Bonet rose to prominence in the 1980s as a main cast member on "The Cosby Show", playing Denise, the teenage daughter of Cliff Huxtable (Bill Cosby) and Clair (Phylicia Rashad).

Though the role brought her acclaim and an Emmy nomination, things were not as rosy on the set of the NBC series. Notably, Bonet and Cosby clashed, particularly over Bonet's explicit role in the 1987 film Angel Heart, her marriage to rock star Lenny Kravitz, and her nude photoshoot for Interview magazine, which created a media frenzy and jeopardized Bonet's role on the sitcom.




Ultimately, the agreement Cosby came to with Bonet was taking Denise off "The Cosby Show" and giving her a spin-off sitcom of her own called "A Different World".

However, once Bonet became pregnant with Zoë Kravitz, Cosby allegedly had her written off her own show. Bonet returned to "The Cosby Show" during its fifth season until she was ultimately, once again, fired by Cosby after the seventh season.

https://www.tvline.com/2061145/most-controversial-tv-show-exits/

Lisa Bonet's final departure from The Cosby Show after the seventh season in April 1991 was officially attributed to "creative differences".

This was the public statement given by Bill Cosby, who also claimed he felt the material was no longer challenging for her as an actress. However, this reason is widely believed to be a professional explanation for ongoing, private conflicts between Bonet and Cosby over her public image and behavior, including her role in the film Angel Heart, her marriage, and her pregnancy.

Context of Departure
Creative Clashes: Tensions between Bonet and Cosby were ongoing throughout her time on both The Cosby Show and its spin-off A Different World. Cosby had a strict vision for the wholesome image of the Huxtable family, which he felt Bonet's independent artistic choices and personal life (such as her explicit movie role and nude photoshoot) compromised.

Pregnancy and Character: Bonet's real-life pregnancy with her daughter Zoë Kravitz was a major point of contention. Cosby refused to write her pregnancy into the storyline of A Different World, which led to her initial firing from the spin-off and temporary return to the main show.

Behind-the-Scenes Issues: Beyond "creative differences," there were also reports that Bonet developed a reputation for being difficult to work with, including occasionally showing up late to work.

Final Firing: After returning to the main show and her pregnancy being concealed through filming techniques, Bonet was ultimately fired by Cosby after the seventh season. She was not invited back for the series finale.
Bonet has largely remained private about the specifics of her exit over the years, though she later stated she "always" sensed a "sinister, shadow energy" during her time working with Cosby.

I just came across this interesting observation (https://www.lipstickalley.com/threads/top-10-times-one-tv-show-destroyed-an-actor%E2%80%99s-career.6093623/post-114487492) about Lisa Benet and her complicated relationship with Bill Cosby:
The thing with LISA BONET is she was the only COSBY KID that didn't REVERE Bill Cosby and allow him to control aspects of her life outside of filming. Bill Cosby had a GOD COMPLEX he loved to be worshipped, and he was used to people doing what he told them to. All the COSBY KIDS pretty much went to COLLEGE and got DEGREES because of BILL.

LISA BONET never gave a damn about trying to please him and honestly, she got into acting by mistake. She wasn't even trying to be an actress thing just took off for her and she got lucky. LISA was bohemian and NO ONE tells her what to do, complete FREE SPIRIT. There were many things that pissed Bill off. Her pregnancy, her marriage, Angel Heart (nudity) and the Rolling Stone cover and the fact SHE WOULDN'T LISTEN TO HIM.

INSIDERS said Bill wanted LISA BONET gone years before, but her character was extremely popular, and he didn't want to screw up the success of his show, so he decided to give LISA a spinoff because she still had 3-4 years left on her contract. He wanted to distance her from TCS. However, after she became pregnant to PUNISH her TOOK HER SHOW from her and wrote her back into TCS.

LISA didn't mind because she got to WORK LESS and spend more time with her BABY. It is very telling in 30+ years LISA has never participated in any of the reunions and fanfare because she literally doesn't care! TCS was JUST A JOB to her.

Bonet didn’t kiss his @ss, so Cosby didn’t like her. She had his TRUE personality nailed immediately.