This is currently the default behavior for linked record fields. Out of the box, records are expandable, but any fields in the linked records are set to read-only. This behavior can, of course, be changed. To not show any details about linked records (i.e. just their title), simply turn off Allow users to expand (or edit) linked records.
If you want to make the record clickable:
Enable the option to Allow users to expand (or edit) linked records.
Go to the child form settings (not the parent field) and make all fields read-only.
When all the fields are set as read-only, the 'submit' button will also be hidden.
💡Tip
This function also makes the records “expandable” in the gallery view, allowing users to "learn more". For example, you can make the record (card) preview some high-level information by default. But when the user clicks, it pops up to show full details.