The ingredients item group contains an area for ingredients that can be used in a brewing stand. Despite stone, slime blocks, and cobwebs being valid potion ingredients now, they are not in this item group. Note that breeze rods are in the item group but listed in a different area. I believe this is a valid bug because it is an inconsistency in the creative inventory.
Steps to reproduce
1. Open the creative inventory
2. Search for 'Stone'
3. Observe that the tooltip for stone does not list 'Ingredients'
Can confirm