Please take this advice.
Look at these screen shots from this VIDIO
With both sockets switched ON the detector finds the Earth is Live, but with only the socket being tested switch ON the Earth on that socket is no longer Live.
Not one of those screwdrivers is it ? If so bin it , they are not reliable .You need a multimeter or a twin probe voltage tester ,minimally ,to do safe testing on live circuitry.
Look at these screen shots from this VIDIO
With both sockets switched ON the detector finds the Earth is Live, but with only the socket being tested switch ON the Earth on that socket is no longer Live.