Strange to say the least. I can understand that cylinder not contributing to the start. To not come on line though as soon as the engine started up is odd. Since from your description the engine always ran better before this episode.

I wonder if in the failure mode that particular glow plug developed a breech to the atmosphere. I have never seen this but strange things do occur. Personally I can think of no other sensible senario but you may find one.

Some people in the past just carried a short jumper when they own a model with the series plugs. Just a test light should identify the culprit to bridge. Actually this is important as many of these cars just will not start without at least some glow plugs operational.

Any failure should be checked also for no short to ground existing in any of the other glow plugs. This would apply too much voltage to the others depending on the one shorted and take it or them out. Usually just one as there is a time factor here where the weakest plug element will open up first. In your case there probably was not another shorted plug.