When filled with rain, cauldrons with a comparator wont output a redstone signal

Place a cauldron with comparators to gather a redstone output then expose them to a rain session, when filled one or two levels of water, comparators wont turn on, when placed a block above the comparator to update it, it did output a signal

What I expected to happen was...
Cauldron will fill a bit and comparator will output a signal

What actually happened was...:
Cauldron fill but comparator did not output a signal

Steps to Reproduce:
1. Place a cauldron down
2. Put a comparator to output a signal when its filled
3. Make it rain, when a cauldron fills comparators wont output a signal