I was having the same issue but I finally figured it out. I also have an Essential 61 and Reaper , running on a windows 10 machine.
First go to Arturia Midi Control center and click on device settings, then on " Daw Fader Mode" , choose " Jump" .
Then in the Reaper Preferences menu, open " Control surfaces /osc/web"
Make sure control surface mode is set to Mackie Control Universal, Midi Input: MIDIIN2, Midi Output: Midiout2, and especially make sure the box that says " ignore Fader moves when fader is not being touched" is NOT checked.
I had that last one set incorrectly as well as the wrong daw fader mode in midi control center. As soon as I changed those, Now i have working faders in Reaper 6.12