We can't give medical advice here so I have to be careful what I say.
As a T1, who injects for everything (no insulin of my own) the amount of insulin I take depends on the number of carbs in my meal (fats and proteins also have an effect but much less so.)
I must admit I used to love bran flakes but as I remember it they are quite carby, potentially as much as your main meal, depending on what you are eating?
Trurapi's action peaks 1 to 3 hours after injection
TRURAPI will start to lower your blood sugar 10-20 minutes after you take it, it has a maximum
effect between 1 and 3 hours and the effects last for 3-5 hours.
So a potential issue is that your meal time insulin may still be in place when you eat your bran flakes? Can you ask your nurse again or maybe try your pharmacist?
How are you testing your bgs? A cgm (libre or dexcom) might be helpful here, as you'd quickly be able to see what is happening...
Good luck