“Some secrets are safer kept hidden. Some secrets are too dangerous to share, even with those you love and trust.” – A Game of Thrones, Ned Stark (page 357)
Bran has not forgotten the cause of his fall. He did not get amnesia. Bran remembers. He remembers while he is still in his coma, while he is with the 3 eyed crow:
Had he always been so thin? He tried to remember. A face swam up at him out of the grey mist, shining with light, golden.
“The things I do for love,” it said.
The crow took to the air, cawing. Not that, it shriekd at him. Forget that, you dont need it now, put it aside, put it away. It landed on Bran’s shoulder and pecked at him and the shining golden face was gone. – A Game of Thrones , Bran (pg 161)
After this quote, Bran gets pecked in the 3rd eye by Three Eyed Crow and wakes up. He pretends he can not recall because Bran knows this is a truth people would be killed over. That he knew without question.
In the books, Bran has nightmares about it afterwards more than a few times. He wakes up covered in sweat, terrified.
In the show and the books, it is clear Bran remembers exactly what happened. In the show, he hangs his head a bit and says he forgets, and even Tyrion can tell in a heartbeat that Bran is lying, that he remembers.
Bran knows accusing the Queen and the Kingslayer of incest and attempted murder of a Stark child is a truth far too dangerous to say out loud.
“In his dream he was climbing again, ……. The gargoyles watched him ascend ….. Perhaps once they had been lions, but now they were twisted and grotesque.
Bran could hear them whispering to each other in soft stone voices terrible to hear.
He must not listen, he told himself, he must not hear, so long as he did not hear them he was safe ……. “I didn’t hear,” he wept as they came closer and closer, “I didn’t, I didn’t.”
He woke gasping, lost in darkness and saw a vast shadow looming over him. “I didn’t hear,” he whispered, trembling in fear, but then the shadow said “Hodor”, and lit the candle by the bedside, and Bran sighed with relief.”- A Game of Thrones, Bran Stark