I would think that it would be an early symptom. Nocturnal leakage post op is pretty common. The sphincters normally relax some during sleep to allow for expelling gas. There can be temporary nerve trauma/damage that is subtle, and this is why it is only appparent during sleep. Weak sphincters could even be present before surgery, but not tested with chronic liquid stool. This is why some people are helped by bulking agents.
Mark, since you are intolerant of psyllium, you may want to try methylcellulose (Citrucel) or wheat dextrin (Benefiber).
Also, if you can eliminate excess gas production, there should be less leakage (because of the tendency to release gas during sleep). This is why antibiotics may help.
But, without evidence of pouchitis or bacterial overgrowth, you just have to assume there is persistent sphincter weakness or nerve damage. If antibiotics help, but there is no pouchitis, then there is bacterial overgrowth.
Jan