Linux - "Keep me logged in" doesn't work

Steps to reproduce

1. Start the Minecraft Launcher on Linux while logged out of Mojang account
2. In this screen, ensure that "Keep me logged in" is checked:
3. Enter username and password as normal and select "log in"
4. Play the game as normal

Behaviour

After a certain amount of time (seems to be  around a day - possibly triggered by a system reboot, though I can't confirm this) I am logged out of my account automatically, despite having checked "keep me logged in"

I would expect that "keep me logged in" would keep me logged in for longer than that, and indeed on my old Windows machine I was almost never logged out.

Any idea what might be causing this?