updateOne doesn't resolve $$NOW and fails validation

Working with Data
,
1

Ok, so this one is a bit weird. According to docs $$NOW is supposed to resolve into current datetime. I have $jsonSchema validation that contains:

"offlineSince": {
  "bsonType": "date"
},

If I try to use this command in mongosh I get an error:

> db.user.meta.updateOne({_id:'652656'}, { $set: {isOnline:true}, $setOnInsert: {schema:1, offlineSince:'$$NOW', dms:{ids:[]}, dmPts:0, groups:{ids:[]}, channels:{ids:[]}} }, {upsert:true})
MongoServerError: Document failed validation

But at the same time the following works:

> db.user.meta.updateOne({_id:'652656'}, { $set: {isOnline:true}, $setOnInsert: {schema:1, offlineSince:new Date(), dms:{ids:[]}, dmPts:0, groups:{ids:[]}, channels:{ids:[]}} }, {upsert:true})
{
  acknowledged: true,
  insertedId: '652656',
  matchedCount: 0,
  modifiedCount: 0,
  upsertedCount: 1
}

The only difference is use of new Date() instead of $$NOW. Why is this happening?