some PFRelation queries fail without calling back

[mathieulb]

New Issue Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.
• I can reproduce the issue with the latest versions of Parse Server and the Parse ObjC SDK.

Issue Description

some PFRelation queries fail without calling back

Steps to reproduce

obj.someRelation.query().findObjectsInBackground(block: {data,error in })

Actual Outcome

Depends on which relation (no idea about the difference) :

• until now, it worked
• for some newly created relation I have, the callback is never called, no error message in the Xcode log, no error message in the server log.
  ** the same query works fine using the Javascript lib (non-server).

Expected Outcome

• call callback with either [PFObject] or Error object.

Environment

Client

• Parse ObjC SDK version: 5.1.1

Server

• Parse Server version: 7.1.0
• Operating system: Heroku 24
• Local or remote host (AWS, Azure, Google Cloud, Heroku, Digital Ocean, etc): Heroku

Database

• System (MongoDB or Postgres): MongoDB
• Database version: 3.6.12
• Local or remote host (MongoDB Atlas, mLab, AWS, Azure, Google Cloud, etc): Heroku ObjectRocket

Logs

[parse-github-assistant]

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