copied from a comment by patreon user James Bailey on Sirs and Madam's reaction to this episode
"If you guys remember back to volume 3 where it showed Cinder starting to absorb Amber's maiden powers, she used a weird parasite Grimm that came out of the glove. When the connection was severed you saw the glove disappear, I'm assuming it was absorbed, and some black marking's appear on her back. I don't think anything was ever confirmed but I believe that parasite Grimm has been inside of Cinder since then. That is what I believe gives her the ability to absorb other maiden powers. That is also why she was so heavily affected at beacon when she didn't even have the Grimm arm yet, and also why it affected her eye and voice when those aren't Grimm parts either."
Bascially Cinder has had grimm in her since she took the maiden power, that grimm part is the "weakness"
Justin B.
2019-10-20 23:34:13 +0000 UTC
When Ruby mentions how her eyes reacted to Cinder, it wasn't because she had a Grimm part. Cinder acquired her grimm arm in volume 4, so it wasn't Ruby's eyes reacting to it. Perhaps it was what Salem was reffering to as the "weakness" of having the maiden's powers.