RetroArch, can't save to state slot 0

Hello all, and thanks for all the work done on the front-end as well as in advance for any help on this.

I am working on setting synchronization of saves and save states between different devices using SyncThing so I can have seamless saving between devices, basically how Steam Cloud works. In doing so, I seem to have found a weird issue.

On my Steam Deck, in RetroArch, the default save state slot is 0, and saving and loading works fine. On RetroBat, the default save slot seems to be 1, and likewise works fine. If I press Hotkey + Down to select save slot 0, Hotkey + Top loads the state synced over from the Steam Deck just fine as I would expect, but if I press Hotkey + Left, it saves in slot 1 instead of 0.

Is this expected behavior? It may be I'm just not doing something right.