Hi Nick,
I have tested this issue and it seems to be a bug in XPToolBar. I have logged a bug report # 2132 in this regard. You can track the progress of this issue at this link below :
Defect #2132 - Problem with Shortcut keys and Menu Mnemonics in XpToolBar.
Thanks for bringing this issue to our attention.We will fix this issue at the earliest and keep you informed.
Thank you,
J.Nagaraj.