See this isn't really true. It's you, and the british government, latching on to some poor wording in Bangladeshi law.
If she hasn't sought out her passport by the time she's 21, she loses her citizenship, which is why the British govt was ruled not to be allowed to do this to some older Daesh men in a previous case. So she's not really a citizen at age 15. Not really. You can't be a full citizen of somewhere only for that citizenship to be lost when you hit a certain birthday. At very best, she had some kind of a provisional citizenship subject to confirmation by her/her parents (which didn't happen).
Hence my use of the word sophistry earlier.