I received a spring and side plate, courtesy of rcathey, today. There was an obvious difference between the spring that I received and the one that I had been using (I couldn't detect any difference other than cosmetics in the side plates).
Attachment 94550
The spring rcathey sent is on top. You can see the right end of the spring has a different profile. The spring also measures .1mm longer. My guess is they started out the same 120 years ago, and the one in my 98 has had a bit more wear.
Anyway, after installing the replacement spring, the last cartridge in the magazine now loads as you would expect. Problem solved. Thank you rcathey for the materials.