Never have experienced. What I'd start with is ensuring all relevant form properties are the same, even borders. If that all looks OK then perhaps use code to control the position of both forms in both situations using MoveSize method. It might be due to one form having a subform as you say. One thing I can point out is that if you save a form while it's in a position, Access remembers that position next time you open it. Perhaps you did that and have inadvertently set its horizontal/vertical position to be somewhere you don't want. That doesn't apply to tabbed form views of course.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.