Correction:

I've never seen a subform filter records unless the Master:Child links WERE established.

Out of curiosity: Are there any relationships, global or other, between any of the fields in the master form and fields in the subform that Access might be inferring from? Even an ad hoc relationship established through a query in a combo box?

Just wondering if maybe there is a cascading inheritance at work such as a global relationship that might get overridden by an ad hoc relationship which might also get overridden by Master/Child relationship property. Working backwards, the last or highest level of established relationship might dominate until the master form becomes unbound with the subform filtering on that higher relationship?

Or, depending on the version of Access you're working with, if it's an Insider build, if there is a cascading combo box that might be establishing some sort of relationship/filter.

Grasping at straws, just some thoughts that ran through my head at first look.

That's an odd one. Access should not automatically apply a form/subform filter just because the underlying tables have a relationship. Table relationships enforce referential integrity and help with joins, but they don't normally act like Link Master Fields and Link Child Fields on a subform control.

I'd check a few things before blaming the bound parent form itself:

1. Make sure you're looking at the properties of the subform control on the parent form, not just the form object loaded inside it. The Link Master Fields and Link Child Fields properties belong to the subform control.

2. Check the subform's RecordSource. If it is a query, make sure it doesn't reference a control on the parent form, such as Forms!ParentF!SomeField, or a parameter that Access is resolving from the parent form.

3. Check the subform's Filter and FilterOn properties, plus any Form_Load, Form_Current, or other event procedures that might be applying a filter.

4. See whether the parent form is opening with a WhereCondition, or whether its RecordSource query has criteria that indirectly affect what you are seeing.

A quick test would be to open the subform by itself as a normal form. If it shows all records independently, but filters only when placed on the bound parent form, then something about the subform control, its RecordSource, or VBA is involved.

Relationships by themselves should not create a hidden master/child link. If you can reproduce it in a stripped-down copy of the database with just the two forms and tables, I'd definitely be interested in seeing the exact setup.