Update: confirmed and found solution: game allows you to open the door a second time. Have to run around initially to get some distance from the shambler, hit the prompt to open the door (see opening animation again) then book away again to the main glyph where the monster will shy away again back to the normal gameplay.
That's what I am doing. Tried going back to the main glyph where Algernon is standing: die in the circle. Tried going back to first activated glyph: die in the first glyph. Try getting some distance away from the monster and activating the second glyph: activate, instantly cornered, die in the glyph.