Cauldrons under stalactites only fill with lava/water if there's only a single stalactite tip below the fluid source block. If the stalactite is at least two blocks tall, it won't fill up. However, the water/lava dripping particles still show.
- Place 2 cauldrons, a single block layer a few blocks above them, and water/lava source blocks on top of the layer above the cauldrons.
- Place a single stalactite tip above one of the cauldrons, and a stalactite at least 2 blocks tall above the other one.
- Change randomTickSpeed to a high value (e.g. 300)
The cauldron under the single stalactite will fill up (in a short time if randomTickSpeed is high), while the other cauldron won't. Both of them will spawn drip particles however.