I'm having this same experience. I followed the STEP SSE 1.0 guide, installing everything noted, but then I've also added a bunch of cosmetic NPC overhauls and some Dark Brotherhood quests on top.
I met with the Dawnguard, then the vampires. Eventually made my way to get the moth priest and make him my thrall. The Dawnguard around him didn't attack. I've now made it to the Unseen Visions with the moths and scrolls part, and the Dawnguard that showed up didn't attack me, either.
I'm not complaining or asking for troubleshooting help, just saying I've got the same thing.