Please break down here below all the needed steps to reproduce the issue
ne_10m_railroads datasetcartodb_id field from the UI#layer {
line-width: 1.5;
line-color: ramp([cartodb_id], (#00e6a4,#a7e369, #e6de00,#ed8d36,#e60042), jenks);
line-opacity: 0.9;
}
CTRL+S)#layer {
line-width: 1.5;
line-color: ramp([cartodb_id], (#00e6a4,#a7e369, #e6de00,#ed8d36,#e60042), jenks);
}

Thanks @ramiroaznar for your report, already fixed (not deployed yet) with https://github.com/CartoDB/cartodb/commit/6d8fecf6e9cbbd2cdf45fb0fdd8132e5fcdb12e3.
This is still happening. For instance, when changing a popup, it resets the style.
We are still on it @ramiroaznar, we haven't deployed anything yet (check status in the builder project board). Well, issue is not closed so..
this is happening to me too.
Deploying to ded02 for acceptance. Build number 1153.
@ramiroaznar fancy passing acceptance?
Deployed in production.
@ramiroaznar 's life will be better from now on.