It might have nothing to do with high dimension or low dimension, CVX will solve them anyway. Maybe it should be the same, we don’t know your problem too well and your data are randomly generated. and please re-format your code in Markdown format (per How do I include Matlab source code in forum posts?) so it be runnable.

you’ll get a different result.
Note that y and x are un-related variables in your problem. and your problem looks simple, I’m not sure if there is a close-form solution to it.