How should I write the config in "item_settings"? I tried to write it like this ["minecraft:lava_bucket", 15, "hand", "core", "{}", 15], but it disappears after starting the game

This config works for me. Verify that the whole setting looks like this:
"Carried Item Temperatures" = [["minecraft:lava_bucket", 15, "hand", "core", "{}", 15]]

Keep in mind that for core temperature, this is applied every tick, meaning your temperature will almost instantly hit 100.

It's working, thank you