I think selfcest is considered ‘problematic’ on a technicality. it’s a kneejerk reaction of ‘but it’s like fucking your twin!’ which means it’s basically a depiction of incest, and in anti-shipper parlance that means it’s exactly the same as actual incest and shows the creator/consumer’s desire for irl incest.
most anti-shipper positions seem to come from a reactionary ‘ew I don’t like it’ place, which they then conflate with a moral revulsion that indicates the thing that grosses them out is objectively bad. @shipping-isnt-morality is the one who put a finger on the disgust-as-morality thing; I speculate that having conflated disgust with morality, fandom antis then become convinced that anything that disgusts them is dangerous (because we value safety over freedom these days, so safe = moral and unsafe = immoral).
Depending on the nature of the selfcest, I feel like it can be anywhere on a spectrum from incest-like to masturbation-like. For example, a person who has one consciousness in two bodies making their two bodies have sex strikes me as masturbation. Whereas having sex with an alternate universe version of yourself would be more like twins-separated-at-birth incest.
But antis probably hate this way of looking at it, because it requires pondering a) exactly what kind of copies they are, and b) what implications that means for whether their sex is more like incest or masturbation. Especially since some canons are ambiguous. For example, random creepy supernatural twins who talk in unison rarely get explored in enough detail to tell what exactly their connection is.