In my survival world I'm trying to make a system that automatically closes a trapdoor above the cauldrons to keep anything from falling in the lava in them. However, I've found that they only fill with lava if there is no trapdoor above, even if the trapdoor is open.

Reproduction: 

1. Make a lava farm using cauldrons and dripstone
2. Put open trapdoors on some of them.
3. Wait and see which ones fill with lava.

Expected results:

All the cauldrons fill with lava

Actual results:

Only the cauldrons without open trapdoors above them fill with lava