If mongos only targets shard0 with a $lookup, and shard0 then acts as a router and targets shard1 for the foreign coll lookup, most data bearing metrics (i.e. bytesRead) from shard1 won't be aggregated back to shard0. Only metrics that are propagated through planSummaryStats (like docsExamined) are collected correctly. See attached test for example of bytesRead not being aggregated correctly, and docsExamined is aggregated correctly.