Found by the new fuzzer SERVER-92675. The BSONColumnBuilder checks whether new objects have any scalars in them. If the previous element was non-interleaved, it will correctly use this to decide whether to start interleaved or add it as a literal in regular mode. If the previous element was also an object, it will likewise correctly use this to decide whether to start a regular mode or not.

However, if the previous object was an array, it will always go into interleaved mode, even if there are no scalars to reference from.

https://github.com/10gen/mongo/blob/7871ee2aa0d3899aa8f6b3826784df96449e5473/src/mongo/bson/util/bsoncolumnbuilder.cpp#L1420