Uploading multiple attachments simultaneously via REST API returns 403 status

I’ve noticed weird behaviour when dealing with attachments: https://developer.atlassian.com/cloud/confluence/rest/api-group-content---attachments/#api-wiki-rest-api-content-id-child-attachment-put

I want to upload multiple attachments, in this case they are uploaded on the same confluence page, but I’ve also reproduced that problem when I uploaded one attachment for multiple pages simultaneously.

I’ve executed 5 upload requests with 400mb file each, the first two requests completed successfully, but the last three returned 403 status along with this message:

{
    "message": "Current user not permitted to use Confluence",
    "statusCode": 403
}

My question is if there is some kind of rate limiting for this endpoint, but even then it should return status 429 and appropriate Retry-After header.