I'm not sure if it's a regionalism. In any case, I think that we use it that way in Andalucía, Spain.
Why do you think that the meaning is not covered in the DRAE?
For me it corresponds to the first meaning listed there
- intr. Estar, detenerse forzosa o voluntariamente en un lugar. U. t. c. prnl.
Sin.: permanecer, estar, afincarse, establecerse, ubicarse, arraigar. Ant.: irse, marcharse, ausentarse, partir.
I've highlighted one of the synonyms: permanecer
- intr. Mantenerse sin mutación en un mismo lugar, estado o calidad.
Sin.: mantenerse, quedarse, continuar, durar, seguir, estar, persistir, perpetuarse, conservarse, resistir. Ant.: cambiar.
I've highlighted another synonym above: persistir
This verb, persistir (to persist) is commonly used in IT to reflect when a value is stored successfully in a database, a non-volatile memory storage.
So it's perfectly fine and understandable using "quedar" when you're not able to retain, to persist in your brain, what you have learned. And that meaning is covered in the DRAE as you can see.