Something I noticed when viewing paintings (let's say up to XIXth century) showing baby Jesus naked (Madonna with child etc.): Jesus is always shown with a foreskin narrowing at the tip, that is, as almost all Christian boys would appear.
Yet Jesus was Jewish, was circumcised, and his circumcision is even depicted in other paintings (one just sees some old Jewish priest bending over him holding some instruments).
My hypothesis is that Christian painters, although they may have heard “theoretically” about Jesus' circumcision, simply did not connect and painted what they knew about and considered normal in a baby boy.
(An interesting comparison to what many American parents consider “normal”, which is to be circumcised at birth.)