One block lava pockets don't generate in the underground of basalt deltas as frequently as other biomes.
To reproduce this issue, locate a basalt delta in the nether, and fly around in spectator mode.
/locate biome basalt_deltas
MC-225522
One block lava pockets rarely generate in basalt deltas
Open
4
(Unassigned)
Confirmed
(Unassigned)
basalt_deltas, generation, lava
1.16.5, 21w19a, 1.17, 1.21
Retrieved
They do generate, although very rarely, less often than other biomes. Can confirm on 1.17 pre-5.