There's been an increase in the number of papers published at PLOS One that have been retracted, but Retraction Watch notes that that is in part due to additional resources at the journal to tackle publication ethics.
Another contributing factor, it says, is that when Elisabeth Bik and her colleagues reviewed some 20,000 papers a few years ago for evidence of manipulated images, a decent portion of those papers hailed from PLOS One because, as it is an open-access journal, it was easier to inspect. Bik tells Retraction Watch that she informed the powers-that-be at PLOS One about 348 publications with questionable images — a lot, she says, for its small staff to deal with.
Retraction Watch notes that the journal hired three staffers last year to handle concerns regarding publication ethics. "The increase in the number of retractions we posted in 2018-2019 as compared to previous years reflects this shift in journal resources," PLOS One tells it in a statement, adding that the increase does not reflect an increase in papers about which concerns have been raised following publication.