This seems to be enforced in the code on purpose:

// Check type of target:
//   struct or
//   map[T1]T2 where T1 is string, an integer type,
//             or an encoding.TextUnmarshaler
switch v.Kind() {
case reflect.Map:
        // Map key must either have string kind, have an integer kind,
        // or be an encoding.TextUnmarshaler.

And the godoc mentions the same:

To unmarshal a JSON object into a map, Unmarshal first establishes a map to use. If the map is nil, Unmarshal allocates a new map. Otherwise Unmarshal reuses the existing map, keeping existing entries. Unmarshal then stores key-value pairs from the JSON object into the map. The map's key type must either be a string, an integer, or implement encoding.TextUnmarshaler.

So this is "working as intended". I am not sure why it is this way, though. It seems like TextUnmarshaler support was added here: #12146

If you'd like to propose that json.Unmarshaler be obeyed here as well, I think that would be another proposal. But it doesn't seem like a bug to me, as it's documented.

Closing, as this is working as intended.

Well, I left the issue open in case OP wanted to convert it into a proposal, and since I wasn't 100% sure that this is what they meant originally (note how the original title said "marshaling", for example).

Thanks. Having looked through the previous discussions the current behaviour does make sense. The example seems counterintuitive but consistency for the two interfaces wouldn't actually provide any new desired behaviour so I'll close this issue.