Why do you think it might not be a genuine maltese cross ?
I have clicked on the stamp image and enlarged it on my computer so that it fills the screen. It is clearly the product of a handstamp.
It is a very nice neat clean four margin stamp with a good strike of a red maltese cross cancellation.
Red ink was replaced by black ink in 1840 for cancelling stamps during the penny black period especially at major offices. Smaller offices would have continued using red ink until the stock was finished so red cancels may have continued for a considerable time into the penny red period.
At major offices red ink pads were in use alongside black ink pads to produce datestamps coloured to show either morning and evening posts. In a busy office accidental use of a red ink pad for cancelling a stamp was probably the reason for some of these red crosses on penny red stamps.
Sometimes a clerk realising his error added black maltese cross cancels on top of the erronious red cancels. (example attached)
This type of wrror still continued after 1845 and I have a nice block of six perforated penny reds which were first cancelled in red and then overstruck in black with the same numeral handstamp.