Here's the log

You close your eyes for a moment and nod at Tokonwa.
You call to Daevryn, and Tokonwa's wounds are healed by a column of silver light!
You can't learn the nuances of rejuvenate since Arch-Bishop Plantumau is immune to your attack!
Arch-Bishop Plantumau is gushing blood.

You can't learn anything if you're fighting a mob that's immune to your attack.

Kastellyn the Devourer of Magic, Lord of Legends

Maybe that should be looked at. Can the code not be manipulated so that you can still learn say rejuvenation but not learn any weapons? I just have a hard time connecting why you can't learn something that has nothing to do hitting a mob that is immune to an attack.

It could be, but spells are actually most of why the code is there. Mobs that are immune to your physical attack are (well, otherwise would be) great ways to practice invoker spells, for example.

When the mobs is immune to your dual wielded weapon, but not your main wielded weapon?

I'm not sure which way you're asking this, but it's supposed to be that if either is immune, you can't learn.