This is twice now in as many days that the wormsinger stalking party event has not given me a wormfeeder rune. It's frustrating, especially when my fireteam member who did NOT have a wormsinger rune to start with ended up getting one from the ogre that was spawned.
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If you have a "key of (whatever)" in your inventory then you won't get a worm feeder rune until you use the "key of" thing from the inventory