Often the case..She’s perfectly healthy and not showing any signs of sickness. This was a while ago. So...
I had one older chicken hen.. *seemed* fine.. every now and then laid a lash egg but also laid regular eggs..
Animals (even humans) will do everything they can to hide any sign of weakness.
I have stored eggs at room temp for months.. unless they get cracked they tend to just get thick inside from evaporation and brittle shelled. It's possible there was a micro crack not noticed that allowed the degradation. Entirely possible!!
Otherwise.. do consider it a possible EARLY sign of something more sinister. I'm NO fear monger, maybe it correlates or not. Maybe her immune system fought off whatever the infection was?? Just brainstorming possibilities, I too am NO expert! Does the white splodge look like cooked egg?
Hope she continues to stay well, and lay lovely delicious eggs!

ETA: what is in there? The fridge, nest, counter top?