As title reads, the recipe just does nothing when you attempt to deposit the bucket of milk into a barrel of water. I assume this has something to do with it being an item recipe that uses a fluid holding item, and to me it seems like it would make more sense to have it be a barrel fluid mixing recipe instead of a barrel mixing recipe. If for whatever reason having milk be the additive doesn't work in the fluid mixing setup it could be done with milk as the base as you are able to place milk in the ex deorum barrels perfectly fine. This was reported from a modpack and has been tested in a mod pack with just JEI and ex deorum installed for verification.
Edit: Apparently milk does not go into the barrels automatically, there's apparently some other mod that adds milk as a liquid that I didn't realize was the case. However, just for curiosity I added a datapack to the modpack we're using that adds a barrel fluid mixing recipe using water as a base and minecraft:milk which is added from some mod, and the recipe results correctly but doesn't use up the milk in the bucket. So my proposal would be to I suppose add your own minecraft:milk fluid which is just used for barrel fluid mixing this recipe and have milk be the base with water added to it.
As title reads, the recipe just does nothing when you attempt to deposit the bucket of milk into a barrel of water. I assume this has something to do with it being an item recipe that uses a fluid holding item, and to me it seems like it would make more sense to have it be a barrel fluid mixing recipe instead of a barrel mixing recipe. If for whatever reason having milk be the additive doesn't work in the fluid mixing setup it could be done with milk as the base as you are able to place milk in the ex deorum barrels perfectly fine. This was reported from a modpack and has been tested in a mod pack with just JEI and ex deorum installed for verification.
Edit: Apparently milk does not go into the barrels automatically, there's apparently some other mod that adds milk as a liquid that I didn't realize was the case. However, just for curiosity I added a datapack to the modpack we're using that adds a barrel fluid mixing recipe using water as a base and minecraft:milk which is added from some mod, and the recipe results correctly but doesn't use up the milk in the bucket. So my proposal would be to I suppose add your own minecraft:milk fluid which is just used for barrel fluid mixing this recipe and have milk be the base with water added to it.