Yep, she's effectively stateless now because her (provisional) Bangladesh Citizenship lapsed when she was 21 but she was 19 when she had the British citizenship withdrawn. All about legal technicalities, I suppose. Ultimately, needs some kind of international court to decide.
That's not the case (certainly for Ireland), you are automatically a citizen of Ireland if 1 or more of your parents were born in Ireland:
From the Irish Gov website:
"3. Born outside Ireland to an Irish parent
If you were born outside of Ireland, you are automatically an Irish citizen by...
It hinges on citizenship, under international law you can't withdraw a person's citizenship if that would leave them stateless. Shemima's parent's were born in Bangladesh so she is also (and now only) a Bangladesh citizen, so far the legal process has agreed with this. Not sure Bangladesh is...