Valid potion items can no longer be replaced in a brewing stand unless that slot is empty.

How to reproduce

1. Place a brewing stand
2. Use the /loot command to insert a water bottle in the first slot

/loot replace block <x y z> container.0 loot {pools:[{rolls:1,entries:[{type:"item",name:"potion",functions:[{function:"set_potion",id:"water"}]}]}]}

3. Now use a similar command to replace this item with another potion

/loot replace block <x y z> container.0 loot {pools:[{rolls:1,entries:[{type:"item",name:"potion",functions:[{function:"set_potion",id:"swiftness"}]}]}]}

4. "Dropped 0 items"
5. Take out the water bottle and re-run the command from step 3
6. Notice that now that the slot is empty the swiftness potion is inserted in the container
7. Repeat this same step with another container block, for example a chest, and notice that items can be replaced even when the slot isn't empty.