Cities of size 1 survival

Because it is not, in fact, a size 1 city, but a size 2. It has grown to size 2 since the turn you attacked it and reduced it to size 1, but you can't see that yet, until you refresh the map view.
See, Civ doesn't refresh city size numbers (and some other graphics too) if you don't click on the map so the camera moves, or unless you center the view on your active unit, click on a city, etc. Same thing happens when you grow with WLTP day. If you start growing a city with luxuries and just hit end turn, and no other events move your map view, you might have that city grow to 20+ but still display 8 or whatever size it was when you declared WLTP day.
 
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