If a model is not making use of the whole context window - shouldn't that be very noticeable when the prompt is code?

For example when querying a model to refactor a piece of code - would that really work if it forgets about one part of the code while it refactors another part?

I concatenate a lot of code files into a single prompt multiple times a day and ask LLMs to refactor them, implement features or review the code.

So far, I never had the impression that filling the context window with a lot of code causes problems.

I also use very long lists of instructions on code style on top of my prompts. And the LLMs seem to be able to follow all of them just fine.