Using the latest Linux app on Debian "apt install keepassxc" Both slots work correctly on this screen:

But when you go to create a new database only slot 2 is visible/selectable in the drop down.

When you click refresh you see Slot 1 for half a second then it switches to Slot 2 in the field and Slot 1 isn't in the dropdown options.

I identified this issue while working on this - #2064

YubiKey 2 with both slots configured was used to reproduce the issue