OK, some light here. If I go to a different layout, then open window, then switch back to the orginaly intended layout then the global value is retained. That suggests a problem with the the intended layout. That would make sense since I've been working on it a good deal lately.
Suggestions on how to remedy this issue?
Are you sure that you are always looking at the same global field? (Any chance you have two global fields with the same name, but defined in different tables?)
Layout corruption is possible. You might run a recover on the file and see what is reported. Best practice is to not use recovered files but to import your recovered data into a clone of an undamaged backup copy if you have one.
You might also try rebuilding your layout. You can copy and paste all layout objects except the field in question from the original layout into the new copy, then use the field tool to add the problem field to your layout.