Oak has the opposite problem as Juniper.
Oak "My grandson's name is what again?"
Juniper "Let me introduce you to your two best friends who you have known all your life!"
Well... Oak's case was a case of game play/story segregation Blue was nameable just like Red/Leaf Bianca and Cheren on the other hand were not.
Yes. Oak forgets, whereas Juniper thinks you forget.
Personally, I put it down to senility and idiocy.