It seems that some changes from earlier versions of DATAMONGO-758 were inadvertently merged into master although there was already a cleaned up version of it.

In Master:
https://github.com/spring-projects/spring-data-mongodb/blob/master/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ProjectionOperation.java#L404

Cleaned-up:
https://github.com/spring-projects/spring-data-mongodb/blob/DATAMONGO-758/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ProjectionOperation.java#L125