Grpc Golang using UnimplementedServer with other embedded interfaces

I've recently updated to the latest protoc and Go plugins with an existing proto 3 codebase, and am running into trouble with the new UnimplementedServer functionality. The struct that is used for the Grpc server already embeds another interface describing the methods implemented by this service. After embedding the UnimplementedServer reference in my struct, I get an ambiguous error from the compiler and it tells me I am not implementing my service method anymore. Is there some issue with the way I have structured the code? Code to reproduce follows, with libprotoc 3.17.3, protoc-gen-go v1.27.1, and protoc-gen-go-grpc 1.1.0:
api/proto/core_service.proto:

package testbed;

option go_package = "internal/proto";

service Core {
	rpc CreateThing(Thing) returns (ThingResponse) {};
}

message Thing {
	string name = 1;
}

message ThingResponse {
	int64 result = 1;
}

internal/core.go:

import (
	"context"

	"testbed.org/demo/internal/proto"
)

type CoreApi interface {
	CreateThing(ctx context.Context, t *proto.Thing) (*proto.ThingResponse, error)
}

internal/core_endpoints.go:

import (
	"context"

	"testbed.org/demo/internal/proto"
)

type CoreEndpoints struct {}

func (ce CoreEndpoints) CreateThing(_ context.Context, _ *proto.Thing) (*proto.ThingResponse, error) {
	return nil, nil
}

cmd/service.go:

import (
	//"context"
	. "testbed.org/demo/internal"
	"testbed.org/demo/internal/proto"
	"google.golang.org/grpc"
)

type MyServer struct {
	CoreApi
	proto.UnimplementedCoreServer
}

func main() {
	mySvr := &MyServer{CoreApi: &CoreEndpoints{}}
	//_, _ = mySvr.CoreApi.CreateThing(context.Background(), &proto.Thing{})
	grpcSvr := grpc.NewServer()
	proto.RegisterCoreServer(grpcSvr, mySvr)
}

Build:

protoc -I api/proto/ api/proto/*.proto --go_out=. --go-grpc_out=.
go build -o bin/svc cmd/service.go
cmd/service.go:19:26: MyServer.CreateThing is ambiguous
cmd/service.go:19:26: cannot use mySvr (type *MyServer) as type "testbed.org/demo/internal/proto".CoreServer in argument to "testbed.org/demo/internal/proto".RegisterCoreServer:
	*MyServer does not implement "testbed.org/demo/internal/proto".CoreServer (missing CreateThing method)
gmake: *** [Makefile:8: service] Error 2

As the log says, 'Myserver' does not implement the coreserver interface.

type MyServer struct {
    endpoint CoreEndpoints
    proto.UnimplementedCoreServer
}

func (srv MyServer) CreateThing(ctx context.Context, in *proto.Thing)(*proto.ThingResponse, error) {
    return srv.endpoint.CreateThing(ctx,in)
}