if i put -6 db for default send levels (prefrences-vst) then -10 is aplied.
if i put -10db nothings happen
win 11
It works on Win10 for me. But i’ve a drop down menu in the prefrences. I can just select 0 dB,-6dB, -12 dB or Inf. If i select -6 dB it will appears -6.02 dB, it’s ok for -12dB. I need to restart Nuendo to validate this pref. Have you restart Nuendo ?
after a few resets, nuendo and computer, right now this function not work never, always put sends at 0 db. 
Solved
you need restart nuendo inmediatly after change value in drop down menu.
You can not continue working and make more sends, because then when reset, maybe in the menu put -6… but in really always go to 0 when add new send.