There is an additional line below "solve,Z" which contains a single dot (.)
This dot is seen in your screenshot but its invisible if the cursor is not in that region as a dot means subscript.
This dot is interpreted as a second variable you would like to solve for and you are given one possible result for that variable - zero.
I don't know how you managed to get that dot there because when I try to duplicate what you did, the expression throws an "emission failure" error. I can simulate what you did with a space followed by a dot, but not with a dot alone as you have it in your sheet.
The effect is similar like you ask Mathcad to solve for Z AND for a second variable "dummy". Obviously dummy can take any value and this is shown if you add "fully".
So the simple answer to your question is "The 0 is the solution for the second variable in the last line".
I greatly appreciate your time and help, Werner.
Regards.