Hello! OK, so I've been investigating this, and haven't been able to reproduce it; let me ask a couple of quick questions.
Does the path change within the program immediately when loading an image, or only when saving and reloading the project?
Can you post the plan file itself for me to peruse?
Where is the plan file relative to the images - are they on the same drive?
Can you reproduce it if you place MasterPlan on a non-external drive and create a new project?