Ice beneath flowing water does not speed up every entity

Flowing water placed on ice speeds up item entities along the current, but not most other entities.

How to reproduce:

1. On a flat layer of packed ice (not normal ice since it might melt), place water using a water bucket.
2. Throw an item into the water current.
   → The item is sped up towards the end of the current.
3. Spawn a mob inside the current or walk into it yourself.
   → The mob or player is not sped up towards the end of the current.

Expected result:

All entities would be sped up by flowing water placed on ice.

Observed result:

Item entities are sped up by flowing water placed on ice, but not most other entities.