My son plays Minecraft Java Edition on Kubuntu 20.04. Up until recently (a couple weeks ago), it would always log him in automatically. Then, it stopped doing this, and prompted him to choose "Microsoft account" or "Mojang account."
Also, in the course of trying to sign in with the Microsoft account, "X" would pop up a zillion messages saying "the password you use to log into your computer no longer matches that of your login keyring." (No, the unix login password was never changed.) I got rid of those messages by deleting (actually moving aside) the file
~/.local/share/keyrings/login.keyring; the next time we logged him into the Minecraft launcher with the Microsoft account there was a system prompt to enter a password with which to create a new default keyring, so we put in his unix login password for that on the basis of the earlier error message. (The new keyring had "Default" in the name; it wasn't called "login.keyring" anymore but I don't think that's important.)
This seemed to work fine, the launcher showed his Minecraft name in the list and after clicking it, he got the normal launcher screen with a big "play" button.
However, I wanted to test further so I logged out of his Linux account and logged in again. After that, starting the Minecraft launcher newly again showed the "Mojang account" or "Microsoft account" choice, i.e. he wasn't kept logged in. Maybe the keyring wasn't unlocked?
Does anyone know how to permanently resolve this? So far as I can tell nothing was changed locally between the time when it was auto-logging him in and the time when it started prompting for Microsoft sign-in every launch.
(Although, one thing that occurs to me is that it's very rare for him to log out of the Linux account rather than just lock it and put the computer to sleep, so maybe there's some step that needs to happen to unlock the keyring after he logs in? But I don't recall ever doing that before, and he's played for months; I don't think two weeks ago was the first time he's signed out the Linux account since starting Minecraft for the first time.)