This seems fixed in iOS 9 - if Safari is in private mode, the 'screenshot' for multitasking/restore is of empty page.
My e-banking does the same thing - as it requires password when switching back into tab, they show just a big bank logo to hide your account history from multitasker.
My e-banking does the same thing - as it requires password when switching back into tab, they show just a big bank logo to hide your account history from multitasker.