the so fallible Bible

[Tonus]

Why would he be afraid of being separated from Eve when he wasn't told beforehand that they would have to leave as a consequence?

Based on what is told in the story, the only thing they knew was that eating from the fruit of that tree would result in death. That may raise its own questions, in that Adam and Eve may not have appreciated what that meant (did anything die during their time in the garden? Unknown). However, Adam's reaction when god shows up indicates that he feared death much more than he feared losing Eve, since he tried to throw both Eve AND god under the bus ("the woman YOU made, she gave me to eat" etc). Eve did the same, though perhaps she was just following Adam's example (the sort of thing you might expect from a woman crafted out of a rib).

So it seems they understood the practical nature of the order not to eat from the fruit of the tree, as it seems that they were pretty scared of dying. Or maybe that was a result of eating the fruit; it wasn't until then that they had the weird reaction of suddenly being so ashamed of their nudity that they sought to clothe themselves. The story is either very poorly written or so many details have been lost to time that it's a string of statements without sufficient content to really understand what is going on. I'm thinking that the writer wasn't terribly concerned with getting the details right, as much as he was in telling an entertaining moral fable.