mongodb is not using an index for a find command - mongodb

Question

I have approximately 40M documents in a mongo collection. There is an index on the location.country field:
MongoDB Enterprise cluster-0-shard-0:PRIMARY> db.cases.getIndexes()
[
{
"v" : 2,
"key" : {
"_id" : 1
},
"name" : "_id_"
},
//...
{
"v" : 2,
"key" : {
"location.country" : -1
},
"name" : "countriesIdx",
"collation" : {
"locale" : "en_US",
"caseLevel" : false,
"caseFirst" : "off",
"strength" : 2,
"numericOrdering" : false,
"alternate" : "non-ignorable",
"maxVariable" : "punct",
"normalization" : false,
"backwards" : false,
"version" : "57.1"
}
},
//...
]
But queries don't use it:
MongoDB Enterprise cluster-0-shard-0:PRIMARY> db.cases.find({'location.country':'ghana'}).explain({verbosity: 'executionStats'})
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "covid19.cases",
"indexFilterSet" : false,
"parsedQuery" : {
"location.country" : {
"$eq" : "ghana"
}
},
"winningPlan" : {
"stage" : "COLLSCAN",
"filter" : {
"location.country" : {
"$eq" : "ghana"
}
},
"direction" : "forward"
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 0,
"executionTimeMillis" : 195892,
"totalKeysExamined" : 0,
"totalDocsExamined" : 39264034,
"executionStages" : {
"stage" : "COLLSCAN",
"filter" : {
"location.country" : {
"$eq" : "ghana"
}
},
"nReturned" : 0,
"executionTimeMillisEstimate" : 99032,
"works" : 39264036,
"advanced" : 0,
"needTime" : 39264035,
"needYield" : 0,
"saveState" : 39503,
"restoreState" : 39503,
"isEOF" : 1,
"direction" : "forward",
"docsExamined" : 39264034
},
"allPlansExecution" : [ ]
},
"serverInfo" : {
"host" : "cluster-0-shard-00-01-vwhx6.mongodb.net",
"port" : 27017,
"version" : "4.4.8",
"gitVersion" : "83b8bb8b6b325d8d8d3dfd2ad9f744bdad7d6ca0"
},
"ok" : 1,
"$clusterTime" : {
"clusterTime" : Timestamp(1629732226, 1),
"signature" : {
"hash" : BinData(0,"piKWDwLDv7FRcnwCe51PZDLR4UM="),
"keyId" : NumberLong("6958739380580122625")
}
},
"operationTime" : Timestamp(1629732226, 1)
}
Do I need to set up the index differently or do something else to get mongo to use the index? I have tried to hint that it should, but it still does a COLLSCAN. While the examples I've shown above are using mongosh, the behaviour is the same in my node app using mongoose.