Yes, I had originally assumed it would be the traditional style (pictured) in which case isolating the mushroom mince/paste/duxelle for herself and children would have been quite difficult and fraught with danger. Much easier to make them as individual pasties.
I wonder if she marked the pastry, like bakeries do with pies?
I'm certain that was the case. Most who bake are familiar with marking or decorating certain baked goods to differentiate one flavour from another, so to speak.
We are lead to believe that there were 6 (or possibly 7) individual wellingtons made, and that only 4 (or possibly 5) were tainted with death caps.
4x tainted for each of the guests, perhaps 1x extra in case Simon changed his mind about coming to lunch.
1x clean wellington for Erin, and 1x extra she claimed she fed to the kids later.