Existing child epics cannot be removed from parent after subscription downgrade

Summary

If a namespace on Gold has any number of Epics that have their own child Epics, those child epics cannot be removed from the parent if the namespace is downgraded from Gold to Silver.

Steps to reproduce

1. Create an Epic within a namespace on GitLab.com that is on Gold.
2. Add another Epic as a child to the first Epic.
3. Downgrade the namespace from Gold to Silver.
4. Attempt to remove the child Epic from the parent Epic.
5. Observe that it is not removed successfully.

What is the current bug behavior?

The child Epic is not removed from the parent Epic successfully.

What is the expected correct behavior?

The child Epic should be removed from the parent Epic.

Relevant logs and/or screenshots

• Video example

Output of checks

This bug happens on GitLab.com: 13.1.0-pre b4144d50e28