I receive a mathematics paper to referee. The results are correct but well below the standard of the journal they were submitted to. I indicate that in my first report, while suggesting several improvements of the authors' results, better terminology, new questions.

The editor gives the authors a chance to revise the paper, and they implement all of my suggestions and acknowledge that the referee has “substantially improved the paper”. I still think that the paper is not fit for that journal and I state that explicitly in my second report, along with three pages of new suggestions for improvement, detailed proofs of well-known facts of which the authors are apparently unaware and explicit examples. I also suggest a more appropriate venue for the paper.

The paper gets rejected, and in the span of two months it gets published in another journal, with all of my suggestions, including the proofs and new examples I suggested, essentially verbatim. The authors give a generic thanks to the “referee” for improvements, but their wording suggests that they're only thanking the referee of the journal where the paper got published and not the referee of the version that was submitted to the previous journal.

Do you think this behavior constitutes an ethical violation?

Edit: Thanks everyone for the feedback! Many people are advising me to "let it slide". Just to be clear, I never intended to take action on this (e.g. by writing to the editor), as I never believed it to be a blatant ethical violation, but only a borderline one, as my original post title made clear. It seems however that the general consensus is that it is not even a borderline ethical violation. I guess that my reaction stemmed from having always been very careful in crediting outside help. For example, whenever some referee provided me with explicit proofs of improvements of my theorems, I would first ask if they agreed to me including their arguments and after obtaining their permission, I would acknowledge their contribution immediately before reproducing or paraphrasing their proof. But since there seems to be no generally accepted rule requiring that, I am now persuaded that there was no violation of any sort in this case.