Extracting Derived Z-coordinate data

You could try How to „extract“ z-values ? to open the attribute-table, go directly into the field-calculator. Within the geometry-functions you will find z

Just calculate z on the geometry: z ($geometry)

Apparently the z function available in the Geometry section of the field calculator works well with points. I was able to find a solution to a similar problem of that asked by the OP by using z(start_point($geometry)) in the field calculator.