On opening a world, functions appear to be reloaded, but the minecraft:load tag is not called.
This would be very useful for very specific situations in which the stat minecraft.custom:minecraft.leave_game does not activate (e.g. game crashes), as it is the only way to detect the map reloading.
Alternatively, a minecraft.custom:minecraft.join_game stat would be great, and far easier!
Is this an issue on 1.12.2?