Cannot attach files to an updated record

I'm using Invenio 3.2. I create a new record using the REST API, POST method:

POST https://127.0.0.1:5000/api/records/

Say the new record gets id 1. Now I can attach files to the new record:

PUT https://127.0.0.1:5000/api/records/1/files/my_file.txt

The new file gets attached correctly. Later, I update my record using the PUT method:

PUT https://127.0.0.1:5000/api/records/1

Update is successful, but attached files disappear. If I try again to attach a file, I get a 404 error:

PUT https://127.0.0.1:5000/api/records/1/files/my_file_2.txt
{'message': 'The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again.', 'status': 404}