Using Stellarium I checked the magnitude of Mercury as we on Earth see it, which appears to be correct.
At its superior conjunction it is at its brightest, e.g. on the last one on May 5 it was -2.42 at 1.325 AU from Earth.
Now I 'move' to Venus, so as seen from Venus on the same date (May 5) from where it is also almost behind the Sun.
But then Stellarium shows -1.71 while being closer (1.028 AU from Venus) and the phase is also (almost) full: 98.8%.
This cannot be correct.
On the next superior conjunction (Aug 16) it is, as seen from Earth, about the same distance (1.348 AU), but quoted much fainter (2.02).
The phase is obviously the same, at any superior conjunction it is (almost) 100%.
This cannot be correct either.