Liang Chen, a member of the Keen Security Lab, revealed the accomplishment on Twitter and announced he would be presenting the pwn at POC 2018.
iOS 12.1 + A12 == the end of iOS war? I will be presenting at POC 2018 this Friday with latest iPhone XS Max pwn.
Unfortunately, we don't expect that Chen will release this jailbreak publicly. The Tencent Keen Security Lab was established in January 2016 to focus on the security research of mainstream PC/Mobile operating systems, applications, cloud computing technologies, IOT smart devices, etc. As part of Tencent Security their research is applied to Tencent's products and technologies.
You can check out the POC 2018 schedule here. Chen's presentation is at 16:30 - 17:30 on November 9th.