Sometimes the Railroad gives synths new memories. After this the synth no longer knows he/she is a synth and is thus better able to blend into society. But a synth doesn't age and after a few decades people will begin to start noticing this. In light of this the Railroad making synths forget that they are synths makes no sense.

90% of the story in this game makes no sense....

That's the spice of the story and in this case, the tale of Railroad.

The story of Fallout 4 is ironic and I love it.

Hopefully the outcome of the Railroad tale will be covered in the next Fallout.