Thank you for pointing put an important omission. I edited my post above after initially posting to say that that scans 0001 and 0002 were borderless (apart from areas formed by hands holding down the paper at the corners) but then forgot to add that scans 0003 and 0004 do have proper borders to them where the paper was trapped under the printing easel.
Also, I don't use any safelight - the room is completely dark when the paper is out of the box so I think we can rule that out.
I did subsequently get a good print of the picture that is in scans 0001 and 0002 so I'm confident filtration isn't to blame. This is why I'm a bit puzzled because the usual culprits don't seem to be the answer in this case!