The law currently makes anchor babies inevitable; they are US Citizens.
The law is unclear, and there is no Constitutional basis. Thus, Congress can and should fix this.
And congress cannot punish people by using a law passed after the fact: ex post facto laws are prohibited by the Constitution. That means that all anchor babies now citizens will be citizens their entire lives. It cannot be taken from them.
People can and do lose benefits, to which they were not entitled, all the time.
You are entirely missing the point. Saying Congress can rectify something does not change the citizenship status of those already U.S. citizens.
The Constitution prohibits ex post facto laws.