Having the same problem with a game with friends. Happens when you get into combat while having natural explorer, which normally resolves if you finish the fight (not always). Although, hitting a passive bug and letting it live or killing it, will lock this issue for a very long time. It's almost as if having natural explorer is giving the player its respective % move speed buff as a negative debuff instead (level 3 natural explorer -50% move speed). Tested it out with friends that don't have the mutation active, go into combat and run side by side, which looks like they are moving twice as fast as me at base move speed.
