Enterprise test> db.version()
9.0.0-alpha0-6af02c94
Enterprise test> db.c.drop()
Enterprise test> db.c.insert([{c:1}, {c:1, a: null}, {c: 1, b: null}, 
Enterprise test> db.c.find()
[
  { _id: ObjectId("6a5f94165f5dc3a4d21d6f67"), c: 1 },
  { _id: ObjectId("6a5f94165f5dc3a4d21d6f68"), c: 1, a: null },
  { _id: ObjectId("6a5f94165f5dc3a4d21d6f69"), c: 1, b: null },
  { _id: ObjectId("6a5f94165f5dc3a4d21d6f6a"), c: 1, a: null, b: null }
]
// Uses COLLSCAN + GROUP pushed down to SBE
Enterprise test> db.c.aggregate([{$match: {c: 1}},{$group: {_id: {a: "$a", b: "$b"}}}])
[
  { _id: { a: null } },
  { _id: {} },
  { _id: { b: null } },
  { _id: { a: null, b: null } }
]
Enterprise test> db.c.createIndex({c: 1, a: 1, b: 1})
// Uses covered IXSCAN + GROUP pushed down to SBE
Enterprise test> db.c.aggregate([{$match: {c: 1}},{$group: {_id: {a: "$a", b: "$b"}}}])
[ { _id: { a: null, b: null } } ] 

// Problem occurs in classic engine as well
Enterprise test> db.adminCommand({setParameter: 1, internalQueryFrameworkControl: 'forceClassicEngine'});
{ was: 'trySbeRestricted', ok: 1 }
Enterprise test> db.c.aggregate([{$match: {c: 1}},{$group: {_id: {a: "$a", b: "$b"}}}])
[ { _id: { a: null, b: null } } ]