Compilation Error when not expected

syntax = "proto2";
import "google/protobuf/descriptor.proto";

package singla;
option java_package = "singla.Address";

extend google.protobuf.MessageOptions {
  optional int32 message_version = 4121;
}

message Song {
  optional int32 id = 1;
  optional string name = 2;
}

----------------

syntax = "proto2";
import "./test.proto"; // doing test.proto is fine

package singla;


message Album {
  optional Song song = 2;
}

Compiling this throws an error about version 4121 getting reused.

Reproduce the compilation using the two provided proto2 snippets, first importing test.proto directly and then importing it as ./test.proto. Compare the compiler's handling of extension number 4121; done means the valid import form no longer reports that number as reused unexpectedly.