Hopper sometimes doesn't suck items above when they are overlaying

Happens in survival too.
A hopper is filled with one item, but still have space for only this item (look at the image). A water current is above carring 2 items, red wool and blue wool.
The blue wool should go to the hopper, but sometimes its so close from the red wool that the hopper ignores it.

To reproduce:
-Make a clock and a path like the one at the image
-Fill one dropper with red wool and the other with blue wool
-Let the clock run for a while and you will see some blue wool going by the hopper even if the hopper isn't full

I've searched but found none like this at the issues list

Update:
You don't need the water, you can do just like the last image. The hopper is still filled with blue wool, but still have space for it.