They have been implying mental issues for those partaking "mistakenly" since at least 2003.
For instance, some might ask themselves: ‘Do I use medications that affect my emotions? Am I inclined to have deep emotions that I could misjudge?’ (par 20)
http://wol.jw.org/en/wol/d/r1/lp-e/2003124?q=partake+emblems&p=par#h=19