Putting an uncraftable potion in a brewing stand and adding gunpowder plays the brewing animation and spends the gunpowder but doesn't turn the potion into a splash potion. Using a splash potion and adding dragon's breath in the same way doesn't turn it into a lingering potion.

How to reproduce:

1. Place down a brewing stand and fill in the fuel slot with blaze powder.
2. Run the command /give @s minecraft:potion or /give @s minecraft:splash_potion
3. Insert the potion you were given into the brewing stand.
4. Add gunpowder or dragon's breath, respectively, in the ingredient slot.
5. Wait for the brewing to finish.

Expected result:

The regular / splash potion would turn into a splash / lingering potion.

Observed result:

The regular / splash potion remained a regular / splash potion.