When converting from older version (1.17.2) to 1.18 some spawners may generate into the previous layer of bedrock (now deepslate) and create a smaller spawner room.

The chunks were first loaded in 1.17 and later converted to 1.18

World settings used:
Seed: -1318688359
World Type: Default
Generate structures: On
Coordinates:

/tp @p 545 -2 51

World created in 1.17, and updated to a version at, or after 1.18.