Here's a skill everyone should learn that most are unaware of: A dedicated instance is a big box, within it sub-instances called Darkness Zones. When you're in a darkness zone, there are dedicated spawn points for your guardian. In the rest of the instance, it's not restricted... continued progress is allowed. As such the game sets spawn points. I know, from your description, the last time you stood idle for 15 secs was when you were doing the puzzles before boss. On normal, they removed the darkness zone. So, if you're in a non-restricted zone/encounter. Stand still, no inputs for 15 secs to set your spawn point. Next time you do boss wait for 15 secs before starting the encounter.