it looks like a bug, will report it to find out if there is a workaround on either engine or itch's side (not many posts about it, yet, but here's one: https://itch.io/t/1158456/html5-local-storage-seems-to-be-shared-between-multiple-games )
For example when I play Oh My God and look at the local storage, it'll show save data fields from Vampire Survivors, so there's even possibilities of save-pollution between different developers. So if you play another Visionaire game on here, you will likely also see our saves there and vice versa. Easiest solution probably would be if the engine developer made the key specific to the game, similiarly to how android apps are named to be unique. The key now is named "savedata" but it could be something like "developer.game_name.savedata" or if you're going with the android convention "developer_url.game.save". Sadly it's out of our hands and we can only make suggestions. (z10)