I sent an XDm 45 in to the factory to repair some inconsistent and abnormal feeding issues. They replaced the magazine spring and the pistol has had no problems since. I asked the rep if there has been an upgrade in springs  at some point so I could purchase more (since the feeding issues occurred across multiple magazines), and I was told that no such upgrade has occurred.

After opening up the repaired magazine I found that it has one extra coil compared to old XD45 and some new-in-package XDm 45 magazines I had in storage from several years ago. Today I purchased a brand new XDm 45 magazine from the store, and found that it also has an extra coil.

This leads me to believe that Springfield has at some point upgraded the spring in their .45 magazine. If this was advertised, I unfortunately missed the memo. Anyone having reliability issues with their XDm 45 should check to see if their magazines have the upgraded springs.