So the "minecraft" namespace works just fine for (non-vanilla) recipes. It's when you have more than one namespace in a datapack that creates the issue. Here are my testing results:

Same datapack, two different namespaces, same file name(same recipe):
	'Encountered an unexpected exception
		java.lang.IllegalStateException: Duplicate recipe ignored with ID _playerdata:dirt_to_diamond'

Same datapack, two different namespaces, different file names(same recipe):
	'Encountered an unexpected exception
		java.lang.IllegalStateException: Duplicate recipe ignored with ID _playerdata:dirt_to_diamond'

Same datapack, two different namespaces, same file name(different recipes)
	'Encountered an unexpected exception
		java.lang.IllegalStateException: Duplicate recipe ignored with ID _playerdata:dirt_to_diamond'

Same datapack, two different namespaces, different file names(different recipes):
	'Encountered an unexpected exception
		java.lang.IllegalStateException: Duplicate recipe ignored with ID _recipes:diamond_to_dirt'

Two unique datapacks, two different namespaces, same file name:
	Works as intended (replaces)

same datapack, 1 namespace with functions the other namespace with recipes:
	'MinecraftServer    Encountered an unexpected exception
		java.lang.IllegalStateException: Duplicate recipe ignored with ID _recipes:diamond_to_dirt'

1 datapack, 1 namespace (minecraft), recipes:
	Works as intended
	
Okay, further testing also shows that trying to replace default recipes in a minecraft folder also produce the same error:
	'Encountered an unexpected exception
		java.lang.IllegalStateException: Duplicate recipe ignored with ID minecraft:armor_stand'