A lava aquifer generated with a cave directly beneath it. At some spots in the cave's ceiling, the lava aquifer and the cave connect. In those spots, the lava abruptly gets cut off and remains floating within the holes in the ceiling.
Steps to reproduce:
1. Generate a Lush-Cave Single-Biome world with the seed "-453190375585913630" in Java Edition Snapshot 21w16a with the experimental Caves&Cliffs datapack enabled
2. Go to 218/-44/277
3. Look at the ceiling towards the southern wall of the cave
Expected result: Lava flowing from the aquifer into the cave
Actual result: Lava remains stationary and requires manual updating
EDIT: After closer inspection, the phenomenon appears to generate at an intersection of lava aquifer + noise case + cave carver.