When enums are stored as string, they were parsed back case-sensitive until v2.12. Since that update, this behaviour was changed which causes issues when multiple enum values have different capitalizations.

Example:

public enum MyEnum {
    AnEnumValue,
    anenumvalue
}

In this example, Mongo will parse the string representation to the same Enum value.

In my opinion, this behavior should at least be configurable.

References in Code:

- src/MongoDB.Bson/Serialization/Serializers/EnumSerializer.cs (in v2.12, the parsing logic has changed by adding a boolean parameter to Enum.Parse())