I'm sure people have already found this, but for those that haven't, I did some exploration of the Inverted Spire Strike. When you first start, if you turn around there's another HUGE section to explore and climb including several NPC battles that pop up around various areas (timed?). I eventually circled to the milk waterfall, crossed it and jumped into a slipstream from the mission. What I found was a large light wall blocking the path and a pretty cool "return-to-mission" portal for those of us that want to explore and still complete the strike.