Is this cannibalism?
Such questions should never be asked without providing the necessary context. I had to Google “eating placenta” to find out why anyone would do such a bizarre thing (source). As it turns out, some believe that doing so possibly can “ward off postpartum depression and increase milk supply” and usually is done by turning the placenta into capsules (presumably taken like medicine).
If there is a valid scientific purpose to this procedure, it would appear to me to be theoretically plausible that the human placenta could be used for such a purpose and that it would not necessarily constitute cannibalism, since the placenta is, once it is delivered after birth, disposable human waste that is being used for a legitimate purpose (treating depression and increasing milk for the baby). The only question seems to me to be whether or not this actually works or is “scientific superstition.”