The simplest way to recreate this bug is by increasing the default_temperature of steam. For default_temperature < target_temperature of the vanilla boiler, it will start producing more steam, as it seems to calculate the production rate by the difference between its target_temperature and the ...

Oh, I did not think about that. Now it works as intended, thank you.

>What did you do?
I added a modded item with a fuel_value, with the fuel_category set to fusion, and a burnt_result item. Then I inserted that item into a fusion reactor.
>What happened?
The fusion reactor consumed the item without returning the burnt_result item.
>What did you expect to happen ...