According to viewtopic.php?t=128130, item signals are supposed to sometimes not select a recipe, even when the user might expect a recipe to be selected.
As far as I can tell, there's no mention in the Lua API of what a mod needs to do in order to trigger this behavior. Could this documentation be added, please?
Mods can prevent item signals from selecting recipes
Re: Mods can prevent item signals from selecting recipes
Thanks for the report. This information is documented on Item/FluidProductPrototype, see here.