Didn't Malta get the cross from the Order?
Alternatively, it gets added to the Spanish coat of arms as a result of the donation, such as the Spanish monarch being rewarded as being an honorary member of the order, or something like that.
The Knights already held Malta as a fief of the Sicilian Crown. They only achieved sovereignty in the 1750's, under Grand Master Pinto.Looks like, but other groups have used it as well.
Instead, maybe Malta is given to the Hospitallers as a "temporary base" (which becomes de facto permanent), but is still considered Spanish territory. Perhaps it is still considered a fiefdom of the Spanish crown.
Alternatively, it gets added to the Spanish coat of arms as a result of the donation, such as the Spanish monarch being rewarded as being an honorary member of the order, or something like that.