Nickel plating blisters when zinc die cast parts are heated to 500 deg F

We are having a plating problem with a ball valve handle. It is die cast zinc, tumbled and then electro nickel plated. When they are delivered they look good and to print.

To assemble, we stretch two Teflon o-rings over the stem. To shrink the o-rings back to size the handle is heated at 500 degrees F for 20 minutes. This is how we have done this for many years with no problems. Recently when the handles come out of the oven most all the handles have bubbles under the nickel plate ruining the part.

The plater claims that nothing has changed in their process. Are there any recommendations as to 1) what the plater can do to ensure this doesn't happen, and 2) are there any restrictions on heating a nickel plated zinc die-cast part?

Thanks in advance