This issue occurs to due to a conflict between linked fields and default values.

If a default value is specified for a linked field in a form and that form is added to an existing issue (manually or via automation), when the form is loaded/shown the value of the Jira field always overwrites the contents of the Form Field. The result of this is that the default value is not respected.

Overall this is desired behaviour as where an inconsistency exists between the contents of a field in an OPEN form and a Jira field, the assumption is that the Jira field is always correct (even if blank). 

To override this behaviour for default values in a form could lead to scenarios where the default value always overwrites the Jira field, which could make it potentially impossible to change the value in the field. 

Workaround: The equivalent of default values in a form can be achieved using Jira Automation and setting the value of Jira field. The value of the Jira field will then be populated into the form.