I doubt this is the actual cause, but I can’t think of any other plausible explanation. One possibility is that repeatedly requesting manual indexing in GSC (Google Search Console), while the same URLs were also being discovered automatically through the sitemap, may have unintentionally triggered a spam or quality signal in Google’s indexing system.
This kind of duplicated or aggressive indexing behavior could be misinterpreted by the algorithm, even if the content itself was legitimate.