It looks like the necessary ingredients to repro are:

• a collation
• multiple point ranges
• the point ranges are indexed in descending order
• more than one index can satisfy the query
• the query asks for a sort within each point range

var coll = db.foo;
coll.drop();
db.createCollection(coll.getName(), {collation: {locale: 'fr'}});
coll.createIndex({x: -1});
coll.createIndex({x: -1, y: 1});

coll.find({x: {$in: [2, 5]}}).sort({y: 1}).toArray();