What the issue is: I've been playing with sprinters enabled. Occasionally, I'll load a game up, and the zombies will be automatically reset to shamblers. This happens more often if I've died and created a new character in that game world, but it isn't exclusive to it.
How to recreate the problem: Start a game with sprinters, die, and create a new character. The zombies should be shamblers now.
Am I playing with any mods? Some map extension mods, and some mods that add objects to the game (i.e. a silencer mod). I'm also playing with superb survivors, but I don't see why those mods would impact the zombie spawn type.