There have been a lot of “miraculous” medals designed that only use part of the design that St. Catherine Laboure had made, or have some aspects of it not correct. For example, some of them have the incorrect number of stars or add/ remove things on the back design. I too have seen some “miraculous” medals where the back design is missing. I presume this was done by the manufacturer to save money on manufacturing them and by allowing the use of thinner metal/ less metal, since you can make it thinner if you’re not going to have a design on the back. Your medal is gold so this would allow a considerable cost savings by using less gold.
It’s generally thought that for practicing the Miraculous Medal devotion, you need a properly designed medal or at least one that comes close (like it might have an extra bar on the back under the M, and even this is a subject of some controversy), due to the belief that Mary herself designed this medal. If this were a cheap medal, I’d suggest taking it back and getting one that is properly designed on both sides. However, this is not a cheap medal, but instead is a loving gift from your dear mother made of precious metal. You don’t want to be returning a mom’s gift and also, it might cost a lot more to get a gold medal that’s designed on both sides. I’d therefore suggest that in this case, considering all the circumstances, you just wear the medal you received from your mom. Mary and Jesus will understand.
Edited to add, was your mom aware that the back of the medal was blank? I only ask because a lot of Catholics I know would feel cheated if they bought a Miraculous Medal and then discovered later it had a blank back, and would probably return it to the store. But if your mom knew about this before she bought it, then not a problem.
