I am not sure whats going on and it may be a bug in Prime.
When I load your sheet I see this

Obviously, as you stated, Prime is not calculating but just returns the guess value. What I see when I load the sheet is the state you saw before saving the file.
BUT when I (without making any changes whatsoever) click in some empty space in the sheet and press F9 to let the complete sheet recalculate, things seems to straighten and I get the correct result:

I also tried to type the whole solve block again from anew. I had to manually label some variables as being "Variable" but then got the correct resault.
So I am not sure why you experienced that strange effect.
Not sure if it has something to do with it but I am using the interims-version 10.0.1.0 of Prime.
BTW, you sure know that you could also use the "root" function to solve equations in one variable:

You sure could also use the symbolic "solve" but I would advise against using it.

Should normally only be used when you really are looking for a symbolic solution, like
