That is an interesting one, have tried to replicate your issue on my box but if performs exactly as you would expect, I hope perhaps a member of the team can help as it would appear to perhaps be an issue with your box as opposed to a bug.
I have also noticed this happening recently. It doesn't always happen but usually occurs the first time I lunch freeview into full screen. If I press the up botton the mini guide jumps straight to BBC1. The next time it functions as normal and shows the mini guide for the current channel that I'm watching.