How I tested; 20 runs of the start of Garden of Salvation, returning to orbit after all Vex at the intro were killed. ~450 kills per run. Results: No engram drops when Warmind Cells were used to kill most enemies, but when doing the same area with only a Rocket Launcher and a Gnawing Hunger, Engrams dropped as expected. When running Altar of Sorrow, I noticed that Engrams were surprisingly rare with my then-new Warmind Cell build, so I believe this isn't just area-specific. There's also the same-old issues with Warmind Cells not damaging enemies spawned after the Cell, as well as the Cells randomly despawning prematurely.