Waterlogged leaves do not flow

Because waterlogged blocks do not flow water out of covered sides.
Do NOT intentionally create duplicates.

Then i guess i dont understand how those sides are considered "covered"?

See MC-249394.

I guess the explanation is just that this is a special feature for leaves (and mangrove roots). As opposed to other waterloggable blocks, they are full blocks and have nowhere for the water to flow out.

If you check out how waterlogged stairs work, you'll notice that it'll work the same: The water will not flow out of full block faces (i.e. the back of the stair block), just the front and the sides. The same logic is then applied to leaves and roots.

I guess I understand why this is the case, but it definitely feels like a bug