That's the entire point of unallocated! It's in the name "un" as in "not allocated". That's the risk you take with unallocated, there is no metal "reserved" for you, it's essentially just there (or maybe not) as part of their working inventory. That's as true for the Perth mint as it is for any dealer. If the company goes bust you have no recourse if there if no metal left (Perth mint is different in that unallocated is also government guaranteed)
Allocated is allocated. If you buy allocated from any dealer (not just Perth Mint), they must set aside a physical bar for you and take it off the company books, you become the legal owner. If they don't do that then that's fraud. It should also be discovered in any independent audit, or you can go and physically inspect the metal yourself.