Is there not another angle to this complicated situation
If she joined the ISIS Caliphate (effectively becoming an ISIS national) and that Caliphate has effectively lost their territory, is she now also part of the ISIS Diaspora?
As a hypothetical example, if the Czech Republic (or any other state) was invaded and completely annexed, which state who the fleeing refugees/ diaspora belong to?
Not in my view. We have, in a cowardly manner, tried to wash our hands of this issue and dump her on Bangladesh (where I believe she's never been) or Syria (a failed state which she essentially went to conquer). There is no ISIS diaspora because ISIS was never a country. It was literally never recognise by a single country and didn't perform many of the basic functions of being a country. She was a stupid kid, who went to a militia's local territory which has now been defeated and is now in a country which has no relation to her whatsoever.
The example does not hold and actually there is no example which holds. This isn't the equivalent of a normal state. The closest equivalents would be saying something like the territory that FARC controlled in Colombia or the warlords in Afghanistan or the Lord's Resistance Army in Uganda if they'd managed to get a bit of an international following like ISIS did. Instead they were mostly local so when defeated, they slunk back to where they came from.
In the example of the Czech republic, the overwhelming majority of the people there are Czech. They speak Czech, they are culturally Czech, their family may well have been there for generations. They don't have 'another' place to go. Either they stay part of this country they've been annexed to or they have to go as a refugee and apply at another country, like most refugees do. Shamima Begum is not Syrian. She is not Arab. I reckon she probably can't even speak Arabic. Culturally, she is not Syrian or Arab. She has no link to that country or region at all and has been there for a grand total of 3 years. I'm sorry, it just isn't the same.
Want to block her from coming back and tell her that she needs to seek asylum elsewhere? That's fine. But revoke the citizenship and say she's now Bangladesh's/ Syria's problem. I don't agree with that tbh.