Where did my CoreStorage partition go?
It is possible that you misinterpreted what you saw. The diskutil list command in Terminal, for example, would show you both the 128GB physical disk, as well as the 128GB CoreStorage logical volume. These are listed separately, but are in fact the same storage.

Running disktuil cs list in Terminal will confirm if you have a CoreStorage volume or not. To my knowledge, it's not likely that a CoreStorage volume would revert itself. One possible cause that I can think of would be a reinstall of macOS.