When a non-default model is attached as a reference internally within its own file, and that non-default model is then renamed, after saving and reopening the file, the non-default model is missing or not found as an attachment
When a reference is attached, there is certain data written to the file. With a normal external reference, the file name and model name are written to the file. However, if the reference being attached is an internal non-default model, the file name is left blank (when written to file) and only the model name is written. Therefore if the file is renamed, the models will still be found, but if the model is renamed, it is treated as a normal external reference and will not be found. If the reference being attached is the internal default model, both the file name and the model name are left blank (when written to file), so the default model can be renamed and still be found.