It's not up in the air. He's been very clear he walked out because he was angry his songs kept dropping off the list. He clarified that Michael invited him to work on the chosen tracks with Teddy, such as Jam, but he refused. From there, he said Michael would call him up to work on stuff and he reluctantly agreed while becoming increasingly upset it didn't go anywhere. Bryan mainly wanted to work with just Michael and seemed to imply Teddy was a plagiarizing punk at the 2016 Paris seminar. It's at that seminar he talked about Garbage being the basis for Can't Let Her Get Away, that Teddy lifted the rift/melody. That was actually our first exposure to the song existing. Nobody was sure if he was telling the truth until last year, but he clearly was. Similarly, at the seminar he claimed Man in Black has additional lead vocals he turned off, though fans with DATs in this thread claim he's lying and it's scratch vox of himself.
Bryan was also very bitter about the techniques used to formulate the sound on Michael's post-BAD albums. He agreed with a fan that the way Michael layered background vocals is his signature that was never credited, that Teddy's New Jack style was nothing new because of his work with Shanice and Vesta Williams, and that most of the rifts/melodies are lifted from grooves he did. There's some evidence for this, like with Verdict and Garbage, but it seems largely exaggerated. He claims he wrote the Black or White rap and did the drums, but never provided any evidence for those claims like he did with Do the Bartman. The Simpsons team had actually asked Michael to write the song for them, but he gave the opportunity to Bryan because Michael was a nice guy.
As Dangerous was coming to completion, Bryan's Arista deadline was also on the horizon. He approached Michael about To Satisfy You, how he said he'd do some numbers for his album. I believe they did 2-3 songs with Bryan on lead for the album and he picked one for the feature. Michael did it because of all the hard work Bryan had put into Decade/Dangerous... that and he also agreed to doing it a year or two prior when Music From the New World was initially greenlit by Sony.
I believe, if I recall correctly, Bryan officially left the Dangerous project just a couple months before the album was finished up. He got more and more pissed off seeing his tracks fall off the configuration, and his reasoning was: what are the executives going to think of him after multiple years and no progress? Michael did in fact realize that, hence trying to get Bryan in on the final tracks for the album, but he only had his ego to blame for that one. What's actually unclear about this situation is if Michael realized Bryan was super upset, because you know, men are terrible at communicating and expressing their feelings. I can personally attest to Bryan being very bad at this, as he blew up on me in DMs once for saying one of his new singles sounded very 90s (he did reflect on this... while making excuses to be fair, and later apologized, but it left a nasty taste in my mouth after all the promotional work I did for him). When Michael reached out again in 1993, we cannot readily discern whether or not it was because he simply liked Bryan and assumed he was still just a friend/collaborator, or if there was an element of feeling bad about Bryan's position (which again may not have been communicated whatsoever). Bryan was STILL producing demos at that point, as Dream Girl and others would evidence. This era was only very briefly mentioned at the Paris seminar and skipped over to talk about how he was invited to New York to work on HIStory (he claimed he sang background on Money for example). He said the fact he was willing to do that showed he was not truly bitter, but more so disappointed and upset about the situation. People have pointed out though, regardless of whether he was bitter, his career at that point seemed very much Michael dependent, as he was still paid for all the studio time, and a job is a job even if his failure with Michael earlier potentially ruined future prospects. There seemed to be some evidence in the early 00s that Bryan's camp leaked some of the tracks from the special edition of Dangerous out of spite when the project fell through, but I can't be certain about those whispers.
Someone here also commented he told a fan he briefly worked on demos for Invincible. It's possible given that he was involved in BOTDF, but for the most part he faded out and got strapped for cash. This resulted in him selling the rights to Do the Bartman for about $40,000. Then he vanished, until popping up to write that letter begging for money saying he'll release the master sessions to some songs. When his singles failed, he disappeared again.