Tasks Related To - Custom Module - Subpanel

We have a custom module containing a 1:M relationship to Tasks module.

On the Tasks Record Layout, this 'relationship field' is available for use to link a Task to a record in the custom module.

On the custom module, Tasks appears as a sub-panel which is expected given the 1:M relationship between Custom module and Tasks module.

The data relationship is represented in the database as a relationship table in the custom module.

We would prefer to use the 'Related To' field on the Task record and have achieved this by extending the parent_type_display dropdown via DropDown editor.

 Using this method, our custom module now appears in the parent_type column with the relevant id in the parent_id column in the record in Tasks table

However, when we look at the Custom module record, the subpanel to Tasks does not display the Task.

Do we need to do further work or is the presence of the relationship and table in the custom module overriding how this would normally work with, say, Accounts > Tasks model.

Thanks

Neil