Monstrous Manual, Mummy entry (Page 261).
"On occasion, perhaps due to powerful evil magic or perhaps because the individual was so greedy in life that he refuses to give up his treasure, the spirit of the mummified person will not die, buttaps into energy from the Positive Material Plane and is transformed into an undead horror"
"Because of their magical properties, mummies exist on boththe Prime and Positive Material Planes".
Van Richten's Guide to the Ancient Dead (Page 8)
"The ancient dead are unique among the undead in that the appear to have a positive-energy component. This does not mean that the ancient dead are good - far from it. Rather they have at their disposal an alien power that is disruptive and inimical to life. Positive Energy might also be the source of the ancient dead's most dreaded attack, mummy rot." (the passage continues on for over a page giving explanations on how all a mummies powers come from Positive Energy as opposed to Negative Energy).
Sometimes there are entries in a monsters "Ecology" entries, like Githyanki eggs or Positive Energy Mummies, where you just have to go "No, I'm the DM, and that's stupid" and move on.