How to re-encrypt a decrypted partition that says "The Core Storage-targetvolume is already encrypted"?
When you say you decrypted it momentarily, it can take from minutes to days depending on the size of the partition and data within it and the speed of your system. The same goes for encrypting. The amount of free space also impacts how long things will take as it needs to encrypt/decrypt files somewhere.

It could be the case that it hasn't finished decrypting it yet.