Tripwire fails to detect a bobber floating on top of the water below it

Using an Amazon Fire TV

Tripwire fails to detect a bobber floating (not with a fish on the line) on top of the water directly below it. It will detect the casting correctly. Removing blocks under the hooks results in the same.

Casting the bobber onto a solid block under a tripwire detects and keeps the hooks activated; without any regard to how the bobber ends up there.