Custom Content requires unique titles when copied using copypagehierarchy

We are experiencing a strange problem on Confluence Cloud.

We are using the copypagehierarchy REST end point to copy a page tree to another space. The page tree contains a page A with a child page B. In the target space, there is no page A nor B, but there is a custom content with the title B. The custom content has the preventDuplicateTitle property turned off.

The copy operation fails with a message that a page with the title B already exists in the target space. However, manually creating a page B in the target space succeeds.