Right after its release, Apple decided to remove iOS 12 beta 7 OTA due to concerns with performance.
It seems that many users who installed it just out have complained about problems of slowness of applications and that has caused the company to withdraw it for review.
Apparently the problems only occur to those who have upgraded via OTA, that is, from the device itself using a developer profile. That is why the version remains available in the developer center but does not appear in the Software Updates within the System Settings.
After the launch of the update a week after the previous Beta numerous users have complained about problems when launching applications, excessive slowness and even blockades that prevented them from passing beyond the terminal blocking screen. These complaints would have been the reason for Apple to remove, for the moment, this latest iOS 12 Beta. These problems only work with the OTA update, so if you want to install it you will have to download the IPSW from the development center and installed through iTunes.
If you have installed Beta, you should not panic either, since according to those who have suffered from these failures, the phone is gradually recovering normality and everything works as it should have been since the update. Apple should launch a new Beta 7 that does not have this problem soon. Meanwhile we can only wait.