Legacy shell does not allow to add explain after the aggregation command (e.g. db.foo.aggregate([.....]).explain(). mongosh allows that but the behaviour is not consistent.

The following aggregation is executed as expected without the "executionStats" option

db.foo.aggregate([{$match: {a:4}}, {$match: {$expr: { $eq: [{$last: "$a"},4]}}}]).explain()

Instead the following aggregation even if we don't have the option executionStats it's executed as the option was there (not expected).

db.foo.aggregate([{$match: {a:4}},{$project: {a: {$last: "$a"}}},{$match: {a:4}}]).explain()

Note that this does not happen if we use the syntax db.foo.explain().aggregation([....])