*puts team DiFabio hat on*
Well mainly because the moment Talia sees Wayne back, it surely must be paramount in her mind to communicate this info to Bane so he can ensure Batman doesn't reappear and get in the way of their plan.
She has the opportunity to do this that very day before nightfall, and before the fiery Batsymbol makes an appearance.
So the only logical explanation left (which you included) is that Talia DID tell Bane, but they were both content in the presumption that Wayne is held at the Kangaroo court, and not worried about him because of that.
However, that is also problematic for me because i'd imagine Bane would want to take care of Wayne at the Kangaroo court himself the moment he learns of his return, either that or the moment Wayne escapes, the mercs would surely make it a priority to inform Bane, at least before nightfall.
*puts team void hat back on*
Your explanation makes complete sense.