Could you provide a rule number that suggests anything like that happens?
13.3 (3. Save Roll) says to make a roll, modifying by rend (which can be more than -1).
Ishlaen ignore this modifier.
It then says we cannot modify by more than +1. As others have pointed out, the biovoltaic rule sets the save to 3+, which can then be given a +1, meaning a roll of a 2+ succeeds.
I’m not sure where you’re adding this separate ‘negation’ of the positives phase, but I don’t think it’s the case. At least not with rend. Other penalties would of course still affect Ishlaen.