Numerous issues, entire thing needs to be gone back through using a character that has NOT already been through all of the other Legion content to determine precursors. Don’t have notes to refer to all of the jumps and side quests I’ve had to ferret out, sadly – best to simply create a test case and use the guide.
However, just encountered this, you will need to reverse two quests in the guide. Champions of Legionfall (from Maiev Shadowsong in Broken Shore) must be picked up (or completed) before Alonsus Faol will offer A Curious Contagion in the Netherlight Temple. I’ll leave it up to you how to sort that out to avoid backtracking overmuch.