some how the key is engaging in a way to keep the cup attached, that is what it designed to do so a rifled barrel can impart spin to the slug via the cup . Buy putting in a Nitro card you break that connection between the key and the cup.
A tell tail way is to look at the bottom of a spent cup you find by the berm. If it has a defining gouge at the bottom of the cup you know the key is engaging the cup.
I was under the impression that you had only a felt......the key would probably still engage positive enough to retain the cup.
The card you are using, if it is an over shot card it is too soft.
A nitro card is much harder and come in differing thicknesses.......they can also be peeled by the layer to custom fit.
If I were you, to simplify your shot column and gain some consistency in grouping I would ditch all felt and replace with hard Nitro cards....use over shot cards above your payload for helping gain height for your crimp with smooth bore shooting.
For rifled barrel remove the nitro card and replace the space with over shot cards on top of the payload
as long as you swap card and felt space for other cards or felts it doesn't matter what combo or where they end up as far a pressure goes, that all weigh about the same and as long as the same space is used then you still safe on pressure