I had the same kind of issue on Facebook recently. In 1990 I travelled on sailing ship Eye of the Wind through the Solomon Islands and outer islands of New Guinea. In the South Pacific men and women go topless. It is just how it is. I recently scanned some of my old slide film images and attempted to post some of those on FB.
Oddly, some were accepted and some rejected even when these were of women too. I do not recall any photos of males rejected. Up to then I had never heard of their "nipple police" or more correctly as you call it "nipple algorithm". All the photos I tried to post were legitimate "ethnographic" studies of village people going about their daily lives in the village and fields. Not only were some images rejected, I was threatened that if it happened again I would be permanently banned from the site. I was asked if I disagreed with their decision and when I clicked yes (anticipating it would flag my photos for review) another page popped up saying, essentially, "tough luck and hard shit old son, due to COVID we are not doing this - so bugger off and suck it up". It seems that even here COVID is being used as an excuse for bad faith processes and arbitrary and authoritarian behavior just like everywhere else at this time. This is not altogether surprising with FB who seem to believe they are now Masters of the Universe but it seems from your experience that Flickr is just as arbitrary, arrogant and buggy as the people and processes at FB. It is particularly egregious at Flickr it seems to me, given we as users have to pay for them to host our images where as at FB it is sort of free (though we pay in another way I suppose by tolerating their relentless adverts). Although some of my images were arbitrarily rejected by FB, I posted the above photos from the South Pacific trip to my Flickr page and had no problem. But admittedly this was at least 6 months ago so things may have changed since then.
At one level though I am not totally surprised to hear that maybe Flickr has gone down this route of restricting content. Perhaps 1-2 years back when searching groups for "women's portraits", "glamour photos" etc. (I was thinking of trying my hand at this a bit more) I did stumble on quite a few groups where individuals were posting outright home-made porn. This was not artistic stuff - it was graphic and biological. In fact, entire groups were for this sole purpose. As a result later, I did hear that Flickr had a blitz to clear these groups and people off the site to prevent it becoming just another means of disseminating porn. I do not particularly have anything against porn in principle myself (I suppose I am a bit of a libertarian though I do believe in some limits to that) but I can readily see why Flickr may wish not to become just another porn site. I can also see why they may wish to use an algorithm to help filter images being posted given the volume of images held by them on their servers. But it is aggravating if their algorithm is too facile to be able to discern the difference between real porn and actual legitimate glamour or other images especially when it thinks that men have women's breasts. At the very least they should have a mechanism for appeal and human review - after all there are legitimate and in my view wholly acceptable artistic images of naked women which I would never think of as being porn. I imagine a poorly implemented algorithm might well flag a photo taken of a nude painting at the Louvre as porn. (Although in this example below I cannot see the female subject's nipples - did they have "nipple police back in Manet's day? )
Edouard Manet - Luncheon on the Grass - Google Art Project - Le Déjeuner sur l'herbe - Wikipedia
In the case of your photo I suspect that there are two things that have drawn the algorithm's attention - the guy's nipples and the swelling of his breasts. (He is a little porky.) I believe that these algorithms look for both the presence of nipples and actual breast tissue. To be honest based on what I saw in the Flickr groups I mentioned if Flickr is serious about blocking actual porn they would do much much better by focusing the algorithms attention lower down the human body where the focus of attention of most of the porn pictures are - and usually in close up and graphic color.
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PS one way around it I have seen on FB if to clone the actual nipples out. Looks weird! But works to avoid the glare of Big Brother.
Also it is a little ironic (and not lost on me given the topic of this thread), that even on this site - RFF, when I typed sh#t in quotes above, this site automatically censored it, changing that word to "****".
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