Protojson handling of field masks navigating into maps

https://google.aip.dev/161 says that for a field of type map (e.g. map<string, string>), we may allow field masks in requests that traverse into the map, e.g.

  // Reviews for the back cover. The key is the author of the review,
  // and the value is the text of the review.
  //
  // Valid field masks: reviews, reviews.smith, reviews.`John Smith`
  map<string, string> reviews = 2;

We use this in some places in our application.

However, I find that the FieldMask protojson implementation in go:

1. Rejects keys that have underscores (we must camel case the keys as if they are proto fields). This causes identification issues for a map with e.g. both the keys john_smith or JohnSmith as the only protojson field mask that will pass validation is reviews.johnSmith which deserialises to reviews.john_smith.
2. The backtick notation (`) described in that API under "Map fields" does not work.

It seems the relevant code is here: golang/protobuf/v2/encoding/protojson/well_known_types.go

I haven't been able to find a specification for FieldMask in protojson that says authoritatively what it should/should not accept. Can you clarify? Are the AIPs recommending things the proto spec can't support? Is there a problem with the go protojson implementation?

Using google.golang.org/protobuf v1.36.5