This assertion is hit for the repro below.

The bitset size is 1, the data block size is 100. We believe the reasons for this is the match filters on the bucket-level meta field, producing a single value. Then the bucket has 100 data items, so we have a mismatch in the bitset size compared to the data size. This $match on the meta field should yield a MonoBlock of the same size as the bucket, containing the value true.