This isn't the first place I've mentioned this, but in actual fact, there is no way to stop water in the spillway from getting under the slab. There will always be cracks, In the very best of circumstances, there will only be cracks at all of those saw-cut joints (control joints). So yes, when you see water dribbling along the slab and then disappearing, so the slab is dry downhill of the crack or joint, that's perfectly normal and unavoidable.First post, but have been lurking (and learning a LOT of fascinating info) for a couple weeks now.
In regards to your post and the photos above - I'd be equally concerned about the dry areas below certain joints between slabs. While some water (at a VERY low speed) might just divert across a joint for a few inches, it's unlikely for it to vanish like that, right? I mean many areas are just DRY.
Is there a wider view of the spillway at a low flow rate like that which shows any other dry areas?