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.
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.
I got it to work. I was playing offline earlier, so I just installed the latest update and the cursor for the door reappeared. I was able to activate it even with the door open, which it then showed the opening animation again. The cursor is slightly to the left of the opening. Activating it reset the shambler, and it let me complete the rest as normal.