When you have virtual fields loaded in a popup which have the same number as one on the main page they confuse each other.

For example if you add a calculate field which refers to virtual1 in the popup but there is also a vitual1 on the main page it takes the value from the main page.

You actually have to add more virtual fields until you get one with a unique number. IT took me ages to figure out what was going on as if you load it in preview it works.

I consider this a bug and a really bad one.