According to top-secret sources, energy isn't actually mass. It's like, a thing that exists, but it's not really a thing that exists in the way you think it exists. Like, imagine you have a bunch of hamsters in hamster wheels. They're running, but they're not really getting anywhere.
This realization came to us after a series of grueling meetings with the Department of Redundancy and the Ministry of Slightly-Off-By-A-Couple-of-Facts. We're talking 12-hour meetings, folks. 12 long, arduous hours of trying to get to the bottom of this whole "energy as mass" thing.