When a sharded collection uses a compound wildcard index with a descending order as a shard key - like {x: 1, 'a.$**': -1}, a shardkey index scan will miss documents containing the min boundary.

You'll find a reproducible here.

Note that compound wildcard indexes are not allowed to act as a shard key index since SERVER-103774.
However, it would be nice to understand why we are having this behavior to make sure we don't have other bugs caused by the same root cause.