When flying through the ground in spectator mode it is possible to come across small, thin vertical caves along chunk borders. While fairly rare, these air pockets seem to occur most frequently along beaches and stone shores, although I found one such cave completely under the ocean (see screenshots). My theory is that cave/ravine generation near oceans/water has been buggy for a long time (see MC-125033, MC-126133 and MC-225344) and that these small cavities are a rare artifact of this issue which hasn't been documented yet. Of note, the buggy caves in the first screenshot generate slightly differently in both 1.16.5 and 1.17pre4.

As of 21w38a this bug can be found at:
Seed: -8677246858415868816
Coordinates: 53 63 -381