Hi Peter, I'll take a stab at answering your question.
Firstly it seems you are doing nothing wrong. If you are successfully performing the studies / pieces as required on the score and moving on, then what you have done is correct.
There are two issues you have identified which I'll go over so you can understand why you should not be worried.
The first thing is sight reading. The better you get at sight reading, the easier it is to go back to pieces and pick up the "tune" quicker, which helps your brain remember. Like reading a book. You read through a book you enjoy and you retain some of the bigger details that you enjoyed, you don't remember it word for word, but then if you was to read the whole book again, you start to remember different bits that trigger in your mind as you progress again through the book.
This is easy because (assuming here) we can all read fluently and so we do not think about the words when we read them, we just unconsciously make the connections between the words and what we remembered and start bridging those gaps.
Now if we come across a word we have not read before, we read it more slowly, maybe pronounce it, look it up to find out what it means. Music is just the same. the patterns could be considered musical words that we are continuously learning and storing away so that if we come across them again we can read them quicker and quicker, and although we are still calling upon our brain to help us identify those patterns, we are relying on the conscious aspect less and less.
Now how do we get better at reading? We practice reading! Sight reading is no different, and is something that you should definitely try and develop as soon as possible, but may not be essential.
Learning to sight read better, is a case of getting music much lower than your skill level to start and reading it for the first time at the piano and playing it as you read it. Play it as slow as necessary so that you can play as accurately as possible and rhythmically correct (that part is very important) Never go back if you get it wrong, and don't sight read the same piece over and over as then you're just memorizing, not actively sight reading.
The next part you refer to is memorizing. Going back to our book example again. A book is broken into chapters, different sections of the book as it progress's through the story, and we use the chapters as check points, we can summarize chapters, know where we are in the story, what's happening and what can happen next.
Memorizing in music can be done the same way, we can stop and start throughout the piece and use phrases or sections as check points to help us memorize better. This is something that can be quite important to practice, which you can do so by either randomly stopping and seeing if you can start again without forgetting what the next notes are, or it can be done by picking a bar, and seeing if you can recall from memory that section and the notes.
Practicing memorizing is the second skill that you can work on, and is usually more important if you're going to be performing pieces where you know you won't have the music in front of you, or if you cannot sight read pieces you want to learn to perform whenever you feel like it.
So this concludes why I think you are doing nothing wrong. Because assuming you are not trying to memorize these pieces and not trying to improve your sight reading, what it seems you are doing is using these pieces to accomplish technical challenges? If you are progressing through them you are improving your technical ability.
Memorizing should be saved for pieces that you actually want to retain and perform, and sight reading should be practiced on pieces much lower than your level.
Topic: Hands and Ears are learning, the brain is not!!