"If a Jew should purchase and circumcise a Christian slave or a slave of any other sect, he shall not keep that circumcised person in slavery. The slave who endured such treatment will receive the privilege of freedom." [CT 16.9.1]
This same law is repeated elsewhere:
"It is Our will that if any Jew purchases a Christian slave or a slave of any other sect and does not fear to circumcise that slave, then the circumcised slave will gain his freedom. It is not permitted for a Jew who has circumcised a slave of the aforesaid class to retain that slave. ...." [CS 4]
In some ways this new law from Constantine is actually permissive because it at least allows Jews to own Christians as slaves, just as Christians are allowed to own Jews as slaves. In the future, Roman law will forbid Jews from owning Christian slaves at all. In the Middle Ages, popes will forbid Jews from even employing Christians as servants, physicians, or nurses.
Here, though, Constantine is merely forbidding Jews from circumcising Christians. Presumably the point is solely about converting Christians to Judaism — even though Christians are of course allowed to convert Jews to Christianity. It's possible that circumcision without conversion would also be punished because it would give the impression that the slave is Jewish when they actually have the "true" religion, Christianity.