In certain edge cases, the game attempts to generate an igloo (likely applies to other structures as well) that will float above the ground due to a cave or cliff cutting off its bottom.
In this occurrence, the igloo wasn't able to generate properly, and the basement area is completely missing.
Linked issues
duplicates 2
Attachments
Comments 3
The issue may be related, but here the igloo not generating the basement is only part of the issue. The other part is it generating almost completely mid-air.
The issue with igloos generating completely or barely floating is MC-91084. As for the basement, it's not being cut off by the caves below it; the igloo just generates with a trapdoor but no basement which is being tracked at MC-238539 as mentioned above already. This ticket duplicates both MC-238539 and MC-91084. Also, in the future, just remember to test whether issues occur under complete vanilla circumstances before creating reports, as problems that occur under modified circumstances (such as Fabric as shown in the provided attachment) are not taken in on the bug tracker. 🙂
Does MC-238539 describe your issue?