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This is a known bug. It's a bit fiddly to track down but I'm currently working on it and hope to have it live with the farming 120 and herblore 120 update later this month. ;)
EDIT: Confirmed that I was incorrect and this is not related to Ravensworn, see comment below.
Given that /u/darkhearted_raven is the mod that responded, and from my own experience, I think this has to do with the Ravensworn trait. Here's what I think we know about it:
- Ravensworn trait is slot 2 only
- Ravensworn arises as a reroll of a different trait
- Mysterious seems the most likely candidate, as it has an unknown function, is slot 2/3 only, and can be found on every animal
Given all of that, I think this particular bug happens in a rare case where the animal inherits Mysterious in slot 2, rolls for Ravensworn, but "fails" in a particularly weird way that leaves the animal with neither Mysterious or Ravensworn.
Sorry, it's not a ravensworn issue because ravensworn doesn't work like that. I do think I've worked out the problem though and am testing it currently.