What is happening is that if an add-in function is the first thing in the string passed to the EVALUATE() string, it doesn't evaluate the string and return the result. It simply returns the string that was passed to it.
To see if your version of EXCEL does this, try the following add-in function:
If that returns a value of "2", you are not affected (yet). However, if it returns:
...then you are affected.
I think I have a fix I can use, but I'm still thinking about whether it's the best way to do it.