I must say I am not confident this will be fixed, as kikoskiki noted, this seems to be by design. My question is 'why?' Why take away a very useful feature of the selection tool by design? Is this an internal coding issue in AC16? Just seems weird (and annoying). Don't mean to be too whiny, but I really want my magnet back!
To restore the Quick Selection function in Hidden Line display mode is an existing request in our defect database (#141137). We added your remarks to that issue.
Had the same problem out of nothing. What I did was opened a new empty project file. Entered 3D mode (magnet was OK), then went back to plan view hit File --> File special ---> merge. Navigated to my file and merged. Vualia - it's working again