There's a recent article published in Research Evaluation analyzing MDPI's self-citation rates. The author argues that MDPI's self-citation rate is anomalously high relative to leading journals in the same field, and that this is indicative of predatory publishers.
The article contains this paragraph:
The high rate of self-citations of the journal Sustainability is coherent with data that the journal itself provided in its bibliometric review over the period 2009–18 (https://www.mdpi.com/2071-1050/10/5/1655), showing that Sustainability ranks first in citing journals (2,496 cites) very much over the Journal of Cleaner Production that occupies second position (658 cites) in this bibliographic review. It is remarkable that in the aforementioned bibliometric study over the period 2009–18, the first 30 positions in the citation ranking were occupied by journals from the same publisher (#3 Energies, #8 Water, #23 International Journal of Environmental Research and Public Health, #24 Remote Sensing and #30 IJGI). Copiello (2019) also analysed the citations and self-citations of articles published in the journal Sustainability in 2015 and found that the journal had a higher self-citation level than expected.
The cited website is actually an article published in Sutainability. The full citation is
Tang, M.; Liao, H.; Wan, Z.; Herrera-Viedma, E.; Rosen, M.A. Ten Years of Sustainability (2009 to 2018): A Bibliometric Overview. Sustainability 2018, 10, 1655. https://doi.org/10.3390/su10051655
The way the Research Evaluation article cited the Sustainability article is clearly different from the way it cites other papers (c.f. the Copiello (2019) citation in the same paragraph), and this oblique style of citation causes the citation to not show up in the references section. I'm guessing that this is happening because the author believes Sustainability is predatory and therefore does not want to add to its citation counts.
I am wondering if this is ethical.
Update: OUP published an "expression of concern" to say that the Research Evaluation article is being reviewed for unnamed "issues".
Update #2: the expression of concern is resolved now, and the above paragraph remains in the article.