@Kevin Colyer
I guess KDE Wallet will also work but I am not sure. You can try it out: https://wiki.archlinux.org/title/KDE_Wallet
Without the solution/workaround I mentioned earlier, the issue also occurs with a normal Mojang account.
I found the solution. You need to install GNOME Keyring.
I recommend to follow the Arch Wiki: https://wiki.archlinux.org/title/GNOME/Keyring
Although it seems like you can skip this step even if you don't use the desktop environments listed there: https://wiki.archlinux.org/title/GNOME/Keyring#--start_step
Did you also follow the other steps in the Arch Wiki like adding the PAM module?