if you place a sponge anywhere but directly against a water block, it will not soak up any water regardless of whether its in range or not
To Replicate

1. create a hole that water can drain into
2. place two water source blocks side-by-side so both drain into the hole after a couple of blocks or so
3. place a sponge near a water source block leaving a 1 block gap between the sponge and source block (nothing should happen if bug holds true)
4. move the sponge 1 block closer to the source block and watch both source blocks drain away.

Attached are before and after pictures of the setup