Not sure why the position of tan(beta) definition changes the evaluation of beta

It's hard to debug a picture. I assume you have chosen to display an assignment as just "=", rather than as ":=". If so, then the problem is that you when you write tan(beta):=, you have redefined the built-in function for tan(). As it happens, since beta does not appear on the rhs of the definition, it's a function that will always return the same value.

If that's not the problem then please post the worksheet (click on "Use advanced editor" at the top right of the edit dialog)