Hello Peter,
Thank you for contact Toolset Support.
I will be happy to help you with your issue.
I have tried replicating this issue on my local install by extending the Post built-in post type with a Child Post Type and setting the Post built-in post type as parent post type but couldn’t get the same result.
To create relationships between post types you can follow the guidelines here:
https://toolset.com/documentation/toolset-training-course/part-8-one-to-many-relationships-in-toolset/
https://toolset.com/documentation/toolset-training-course/part-9-many-to-many-relationships-in-toolset/
That said, I could be viewing the problem from a totally different angle from what you want to describe. If so, detailed screenshots with a flow chart explaining what you are attempting to achieve might help clear out any doubts from either side.
As a precaution, could you please perform the below steps and let me know the outcome?
Minimal Setup
If you disable all non-related Toolset plugins, switch to one of default themes, does the problem persists?
NOTE:
+ Make a backup (both files and database) for your site before updating/proceeding anything (Always a good practice!).
Should the above not resolve the issue, you could provide us temporary access (wp-admin and FTP) so that we can verify your settings and guide you to the correct path.
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Note that:
+ Backup (both files and database) your site before giving us your credentials
+ It would be better if you give me the test site rather than the live site.
We'd like to request the permission to disable, enable and install themes and plugins for this site. Please accept this?
Awaiting your reply.
Kind regards