Hi, for over a week i didn't upgrade any traits in meso because i ran out of mutagen needed to update the traits that were important to me, during that time the only trait i got from the mystery trait bonus was always the same one, it went on for a week.
then i updated some cheap trait just to test and after that the mystery trait gave me a different card.
#Mystery trait in meso doesn't change unless you upgrade another trait
light wagon
lost berry
alo any dev here, i have the same problem as this one 🙂