How to reproduce: See title, no second player required when joining a server.
How to avoid when modding: Do not store game state in local variables.
What happens: fires in ground_item_fire.lua is local, but should be global.
Solution: add prefix and initialize to empty table in on_init, a migration will be needed, for people coming from lower mod versions.
I've made a temporary mod to fix this: https://mods.factorio.com/mod/ArsonTempFix
