Strongholds are now supposed to be fully encased in stone, including the floor, if I remember correctly. However, a ravine and a mineshaft both intersect and expose the stronghold, and in this instance, removes the floor from one room entirely.

Seed: -2222233814385959079

To reproduce:

/execute in minecraft:overworld run tp @s 1338.17 10.87 971.20 -948.80 5.85