As an undergraduate I wrote and published this paper (on a PDE with GR relevant content) in the Rose-Hulman Undergraduate Mathematics Journal. I'm now a PhD student, and while I thought the result was not bad and fit decently well into my mentor's research program, I thought the writing could use some major improvements. I never posted it to arXiv, so I had the idea to rewrite the paper and post it there.
I submitted this updated version of the paper to arXiv. arXiv placed it on hold for three weeks, and then rejected it on the grounds that it "does not contain sufficient original or substantive scholarly research". They also said they would consider an appeal "if and only if this manuscript (or a revised version) is accepted for publication in a peer-reviewed journal that provides rigorous review and revision". This was a bit frustrating because I have no intention of publishing the paper a second time, but I thought OK, the paper went through multiple rounds of refereeing at RHUMJ, maybe that's sufficient.
So I made an appeal, identifying the prior publication and explaining why I thought the research had value. In particular, an arXiv preprint written by my mentor cited my paper and applied its results. Within a couple hours my appeal was closed, with the explanation that the material would only be considered if it was published in a conventional, peer-reviewed journal. It is not clear to me if arXiv does not think RHUMJ meets these criteria, or if they missed my explanation that the paper was already published.
This brings me to my main question. Could the fact that my paper was a rewrite of an already published paper have been a reason to reject? Has anyone else had similar experiences? I have gotten a few other preprints on arXiv, and I genuinely thought this was a decent paper. My second question is do undergraduate journals like RHUMJ meet arXiv's standards for rigorous peer review?
Technical details: As a short explanation of the research, there is a toy model for a spatial slice in a relativistic spacetime under the influence of gravity and electromagnetism, and the paper shows existence and uniqueness for a PDE related to the problem of a prescribed mass and charge distribution.
tldr: arXiv rejected my rewrite of a result I published in 2017, frustratingly stating that the rejection could only be reconsidered upon publication