When you start to type an expression, it creates an autocomplete list which now includes variables which are declared. While all of the other results ignore the case, the variables shown are displayed by the case of the letter(s) typed. I suggest either making it so it either ignores the case or give the user an option to set a preferred method.
In this example “apricot” only shows up when a lower case “a” is entered while “Apple” only shows when an upper case “A” is entered.
Users use different variable naming strategies; some strict and some very loose. Having to switch between cases is frustrating and can slow you down as you type in one case, delete it and then retype it in the other case.