Retargeting with -Z forwards makes arms go up?

I import a model with negative Z as forwards (no idea why, they just built it that way!). So in retargeting I tried preview - the character walks backwards with knees flexing the wrong direction. So I tried changing “forward” from “Z” to “-Z”. The character now walks forwards with legs etc correct, but the arms are pointing up in the air! Could this be a Machinima retargeting bug?