Castoria might be akin to Nero in the Rome Singularity. Where she is from this lostbelt(would be strange if she wasn't). But I feel like his absence, plus the heavy hinting that there is a Beast being nurtured in this lostbelt, it stands to reason that the only reason Fou would make him self scarce during THIS lostbelt specifically, would indicate that it was able to sense on an instinctual level, itself or to be more accurate, his OTHER SELF, and would likely know that this would be a two way street if he came with us, which would mark the DEATH of Fujimaru right quick, cause it's like being able to sense the other would be like a big neon sign for his other self. Plus oh dear..... this Cath Palug has had MUCH more time to amass energy and likely has reached maturity unlike our Fou.... Oh dear.... oh god.... Oh.... we are in for a very bad time in Part 2.....
"However, as he wasn't yet an adult of his species" by the time of Solomon singularity