Summary

We have multiple  filters with $expr in a list, and we are passing this list in to method
AggregateIterable<TDocument> aggregate(List<? extends Bson> pipeline); of class MongoCollection.  
 
If we have and filter in it and if you see com.mongodb.client.model.Filters.AndFilter it will l combine all the condition into one condition.
 
Value passed as :

{"$expr": {"$eq": [ {"$strcasecmp": ["$CreatedBy", "tenant21"]}, 0] }}

{"$expr": {"$in": [ {"$toLower": "$a"}, ["a"]] }}

 
Command executed:

{"$expr": {"$eq": [ {"$strcasecmp": ["$CreatedBy", "tenant21"]}, 0], "$in": [{"$toLower": "$a"}, ["a"]]}}

If you see above it will combine two $expr into single $expr and while executing it will fail saying, $expr can only contain single object.