Plutarch is often grinding his own axes; one of those “elusive” articles notes that Big A had three different tombs, the final one of which was in Alexandria. Sounds like an ancient precursor to the remains trade, eh?
But only the fruitcake from Athens says his tomb was in Siwa.
I don’t see where Plutarach would have any axes to grind in reporting that the tomb was in Egypt and moved to various locations. That is exactly the type of anecdotal evidence that is found in all his ‘lives’ . It’s unlikely that he’d lie about something that should have been well known to his audience and would have been pointless to lie about.
So I’m going with Alexander’s tomb in Egypt and probably in Alexandria until looted/destoyed.