I was running an echo last night that spawned the Husk of Pannion (I believe that is the correct name) boss. I was kiting off screen while he performed the line of fire attack where he spins in a circle. When I closed in on him again, he suddenly teleported off the screen.
The map showed his location as just behind a wall in the same area. If I approached the wall, he would attempt his attacks through it (some reaching through), but was unable to be targeted. Area of effect skills (Smelter’s Wrath) were unable to strike him as well.
The only way to leave the echo was to summon my portal and lose progress.