Are you using any mods/cc in your game ? Did you do any specific interaction when the pet lost the trait ? Do you choose the same trait as before ? If so, have you tried changing it to a different one and see if it shows up (just for testing)
What was the specific trait that dissappeared? I do pet challenges and i've noticed that when I breed if one of the parents have the "Smart" trait the puppies tend to be born with only 2 traits and I cannot give them a 3rd trait (even in CAS) as it dissappears as soon as I enter live mode again.
I'm not sure if the "Smart" trait is the reason but I might try and recreate the bug later to see if there's anything else in common which could be causing it.
