There're a couple of questions in the OP:
No, it's not normal. Most papers are not rejected after they are accepted. However, it can happen. One relatively common reason for this is plagiarism detected after acceptance. Similarly, other kinds of academic misconduct can result in such rejection, e.g. if one of the co-authors write in saying they didn't consent to their name being on the paper.
However, it's also worth pointing out that many papers are retracted after publication. In terms of the end state, this is effectively the same as rejecting it after acceptance. It's common for papers to be retracted if their validity is contested. Wrong papers will often stay published, but if the flaws are such that it should arguably never have been published in the first place, then retraction can happen. See Wikipedia.
- Can a journal reject a paper without giving the exact reasons as to why it was done?
Journals can pretty much do whatever they want - nothing is published without the editors' consent, after all. But it's rare to reject a paper without giving reasons. Usually the editor will say at least something, even when the paper is desk rejected without review.
Finally:
The paper in question was being held back following an “independent internal review of the editorial process” which threw up unidentified “issues” with the peer-review and revision process, but they didn't specify what they were. The non-transparency is a cause for concern and seems to be very unusual.
This is pretty normal whenever there's a peer review failure. (What is unusual is there being a peer review failure in the first place.) In this scenario, the editor/publisher will investigate what exactly happened. Questions such as who handled the paper, who reviewed the paper, what exactly did they say, are they qualified to review the paper, etc. are all critical ones to answer. An "independent review" of the editorial process generally means someone who isn't involved in the original peer review now looks over the editor's work to see if it's up to standard. Since "issues" were discovered, the original work was probably not up to standard. The result - the paper being rejected - is a sign that in the new person's opinion, the paper should not have been published in the first place.
As for transparency: it's common to not reveal what exactly happened in the peer review process. Peer review is anonymous for a reason. If you reveal what happened, then you break anonymity, with a variety of possible results. You might be able to reveal some details (example) but if the people involved don't want to be named, then their identities really need to be kept under wraps.
