Still happening 5 months later.... with AutoCAD LT 2016 for MAC. Same iMac / OS X 10.11.4. 

PICKADD = 2

Tried changing it to 1 then back to 2, no effect. Minus "-" sign shows above cursor selection box but Shift-Click will not deselect lines/objects. This is very annoying how it'll suddenly start acting up.