I think it's very likely that the day 18 egg started developing on the 16th but did very little growing until the 23rd. It evidently didn't die during the unheated period. And my opinion is that it is very close to hatching. I would not candle it any more for a few days. It needs constant humidity to keep the membrane from becoming too tough for the chick to tear. The egg is very porous at this stage and really shouldn't be handled much. The other eggs are far enough along that you can go into lockdown. I often have eggs that are 1 or 2 days behind the others and they all go into lockdown at the same time.
Good luck and keep us posted.