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it seems to me according to your experience they put you for a rescheduled oath ceremony by a final notice if you missed the first one No matter what , it is kind of like a default ....for them.
I won’t be surprised as most of the things are automated these days.
Back in 2015 when i applied for my own citizenship, they asked me to appear for citizenship test even though i already passed it. Even the agent pver the phone were puzzled on the letter. sonyea anything can happen
 
Here is the update. We received another final notice to Appear for oath and my mother has successfully taken the oath today.