I bought Minecraft for my son and I've been extremely frustrated by this requirement and that there's seemingly no way to stay signed in— I've had to print out my MS account details and tape them to his computer so he can sign in whenever he needs.
I'd recommend using a launcher, it stops that happening. I use Prism Launcher and after the initial sign in about a year ago I've never been asked to log in again even after going a few months of not playing it.
I've worked around this by printing the command line while minecraft is open with something like:
cat /proc/$(pgrep java)/cmdline | sed -e "s/\x00/ /g"
And then pasting that sucker in a script wrapped around a .desktop file and from then on only launching that. Opens just as fast as it did back in 2010 and I don't need to open the launcher unless I need to update the game or something like that.
I only play on single-player, haven't tested on multiplayer. The tokens might expire after some time and thus require you to "reseed" the command line from time to time.
Fwiw I think standard 20.04 already has gnome keyring. As suggested here, try a launcher perhaps? I think the one in the store has been working fine for me.
I'm using the official Minecraft launcher, likely from the snap store? In any case, knowing that this is fixable, I will take a closer look when I have a chance.
Ironically, for all this trouble, at least we were able to get Minecraft running at all on Ubuntu— on a separate Windows 10 machine I couldn't get it going at all. Might try again at some point with a VM so that we can play together.
