Milk adds organic residues to a pot without providing controlled nutrition. Keep houseplant watering simple and choose an appropriate composting or disposal route for leftovers.
Uncontrolled residues are the problem
Milk is not supplied to roots as a measured plant feed. Adding it to a pot can leave organic residues whose breakdown changes the conditions around roots. The effect cannot be predicted from a fixed homemade dilution.
Fermenting does not certify suitability
Storing milk for a set time does not establish its nutrient availability, hygiene or fit for a particular plant. Do not describe the result as harmless simply because it no longer resembles the original liquid.
Choose a better routine
Dispose of unwanted milk through a route appropriate to local guidance; many basic backyard compost systems exclude dairy. Water plants with a suitable source and feed with a labeled product when needed. If a pot is already unpleasant or stagnant, inspect its drainage and root zone rather than adding another scented ingredient.
Related reading: Expired Milk and Plants: Use a Better Disposal Route.