I've created a compound index:

db.lightningStrikes.createIndex({ datetime: -1, location: "2dsphere" })

But when I run the query below the MongoDB doesn't consider the index, making a COLLSCAN.

db.lightningStrikes.find({ datetime: { $gte: new Date('2017-10-15T00:00:00Z') } }).explain(true).executionStats

Ps. When I create a compound index by datetime and another type of field it works (searching by index), as below:

db.lightningStrikes.createIndex({ datetime: -1, source: 1 })

I've seen in the documentation and I haven't found any reason for it.
https://docs.mongodb.com/manual/core/2dsphere/#create-a-compound-index-with-2dsphere-index-key