The Cupertino-based company just stopped signing both iOS 12.1.2 and iOS 12.1.1. Thus Downgrade to a previous version of iOS 12.1.3 is no longer possible, version that Apple is currently signing. Apple stops signing older versions of iOS to force users to update their devices to newer versions.
This move by Apple is focused on improve both overall iOS security and stability, since all the new versions, introduce improvements in both senses. In addition, and although the jailbreak is no longer so fashionable, it prevents the community from continuing to develop exploits for old versions of iOS, this community being the one that is most often affected by these movements of Apple.
iOS 12.1.2 was officially released in December to correct the different errors that arose when using the eSIM, but in addition, Apple took the opportunity to solve the patent problem that had been presented to it with the Qualcomm lawsuit in China.
iOS 12.1.3 was released in its final version two weeks ago, with stability improvements in both Messages and CarPlay, and is currently the only stable public version that Apple offers to all users who are forced to restore their devices in case they have a problem on their device that forces them to do so .
Currently, Apple is testing among developers and users of the public beta, iOS 12.2, the next big update to iOS 12, coming with a host of new features like new Animoji characters, HomeKit-compatible TV features, support for Apple News in Canada, and much more. But while the final version of iOS 12.2 is released, it is more than likely that Apple will release iOS 12.1.4 to restore the functionality of FaceTime group calls.
In fact, there are already indications that there will be a jailbreak for IOS 12, that's why they stopped signing them since the exploit they discovered will work until IOS 12.1.2
I thought the same because just on Monday I Downgraded to ios 12.1.1 and yesterday it was no longer signed