Map reset on death in boss fight but still had portals

Was running an irradiated cleansed map with boss, delirium, and expedition on it. Tier 15. Finished the delirium during the boss fight, delirium dissipated, rewards dropped and I ended up dying during the fight. Never picked up the drops.

When I respawned at the checkpoint before the boss fight the orientation of the map had changed with no map discovery from where I came in, no monsters (there were plenty left) in the map at all, no items at all on the ground, and only the boss to fight.

Not sure what happened but that was very strange.