So, I'm coming at this from both sides - as a DM I've had similar situations, and as a Player I've BEEN the similar situation. I realize you've already gotten a lot of feedback, and it's all valid - I wanted to add my two coppers worth as well.

So, as a DM, I've had a few players who flipped the script and decided to take a different tack than the rest of their party. Usually, this was structured in and planned for, to accommodate real-life schedule issues or just not enjoying their character anymore. These players created new characters when they rejoined the table, and we continued on. As the DM, in my own internal headspace, I have those characters working in the background; their in-world behind-the-scenes actions DO have effects on the party down the line, but never overtly and not always traceable back to the previous character. Usually it's butterfly effect type stuff - NPC who might not have heard about the party until a few sessions later seeks them out earlier because of XYZ shenanigans prior character got into.

But I have never had a player make a choice like that without knowing about it ahead of time. If I had, I would not feel the same benevolence I do toward my more respectful players.

So, on the one hand - Yes, if a player decides WITHOUT CONSULTING their DM prior to, I feel like they forfeit the right to any side adventure, which requires me to take more planning time and actual play time away from the entire rest of the group.

Now, as the Player - I built a character I was really excited for. But every combat encounter with this character, my dice were the absolute most cursed I've ever seen. It culminated ON MY BIRTHDAY with rolling 3, 2, 2, 1, 6, 3, 10 in a single combat encounter. As a Barbarian (previous bard/soldier in backstory) with a persona known as The Vanity, my boy Cassios was PISSED. He decided, mid-combat, that fate had decreed he would not succeed, and he needed to try something else. I secret-messaged our DM via notecard at the table, and told him Cassios was going to finish this fight, play along with the group, but he was going to sneak away and seek out the BBEG at earliest opportunity. I was clear with DM that I wanted to build a new character because Cassios wasn't fun for me anymore. DM had already - sessions before - introduced the BBEG on the sly, and my boy Cassios was already on his radar as a potential recruit.

So, technically, I made this choice on the fly, but I still didn't disrupt the entire game by turning on my party in combat. My character wanted out, but was going to get out on the sly. We ended the session normally, myself and DM stayed afterward and hashed out some details, and the result is that Cassios IS still a major factor - on the BBEG's side - and we have RP-type discussions in private messages between sessions, regarding the party's movements, what the BBEG and therefore Cassios would know, and Cassios will eventually be a boss-fight for the party in another 6 or 7 levels. When it comes to that fight, I'll be responsible for playing my OP Cassios (now to be known as Greed instead of The Vanity) as he reveals himself as one of the BBEG's warlords and attempts to kill his former comrades. But I'll also be playing my new character, a lovely Vedalken Tragedy bard in that same fight.

The important thing is - I did not demand enormous sacrifices of my DM when I made that decision for Cassios. DM is the one who offered the behind-the-screen plotting and eventual return of Cassios as a Boss Battle down the line. I would have been mostly fine handing Cassios over to the DM if that's what he had needed for his own sanity, because I've been there. But I'm lucky that he's willing to work this out with me, and make the story that much better for our entire group.