Macro (#define ...) are only text replacement.
With this version:
#define ALPHA 2-1
#define BETA ALPHA*2
The preprocessor replace BETA by ALPHA*2 then replace ALPHA by 2-1. When the macro expansion is over, BETA is replaced by 2-1*2 which is (due to operator precedence) equal to 2-(1*2)=0
When you add the parenthesis around the "value of ALPHA" (ALPHA doesn’t really have a value since macro are just text replacement), you change the order of evaluation of the operation. Now BETA is replaced by (2-1)*2 which is equal to 2.
