Lingering potion doesn't work on stairs

When mob standing on stairs, lingering potions will not affect them.

What I expected to happen was...:

The lingering potion must affect mobs on opaque blocks.

What actually happened was...:

Instant damage lingering potion, when thrown on the block near the oak stairs does not affect on the oak stairs on mobs.

Steps to Reproduce:
1. Place any block near any stairs block, then throw lingering potion on that block.

2. Throw lingering potion straight on the stairs block.