No problem.
We will need to make a reverse relationship.
Edit the relationship to create a new Summary field of the Maximum Record ID#. Call the field [Record ID# of last location].
Then make a new relationship where 1 Location has Many items.
On the right hand side use the field you just created as opposed to the field [Related Location] which it will suggest. Do not bring in any lookup fields.
Then edit the relationship to bring down the location. Call it [Most Recent Location]
Now you have the location of the most recent Location on the item record. Look that up down to the Locations table.
Now make a form rule on he Locations form that says
When the location is blank
Change the value in the field location to the value in the field
[Most Recent Location]
