Hi John, welcome.
That’s a tough one. It sounds like you’re confident the CGM is reporting the correct figures as you’ve caught it on a BG sometimes, otherwise I’d be asking if you’d checked it was telling you the truth. I would continue as you’re doing, and would probably try reducing my basal a bit earlier, and then by a greater percentage if that didn’t work.
Thinking about causes, given she’s 11, do you think it could be hormonal? Puberty caused massive BG changes for me. Is it happening all the time, or at certain points in the month – in young girls you sometimes can see the BG impact of the menstrual cycle months before it has started.