Minimize with a parameter - bugs in 15 and Prime but different

What you show is not a bug! You are minimizing by changing x AND a and so you always end up with a=x. Obviously they meet at the average of the two guess values (x=0 and argument a)

It seems that in Mathcad 15 the function definition and the definition of the guess value must be done at worksheet level and cannot be made inside a program.

Thats a sad/bad limitation - perhaps you may call it a bug,

Valeriy Kofanov had shown how it could be done that way:

As we can see from Valery's screenshot this limitation/bug is not present in Prime.

But using Prime Valery ran into anther, in my opinion worse, limitation of Prime.

In Mathcad 15 and below we can define a function in more than one variable and could omit the last argument(s) creating another function. This was very useful and convenient when we parameterized a solve block, etc.

This is not possible to do in Prime

and basically thats the error Valery experiences in Prime when he tries to minimize a function g with 2 arguments a and x for just one of them.

You decide if you call it a bug or just one of the many "limitations" in Prime.

Here is a very awkward way to do what Valery tried in Prime: