The bug is in the TcWebHooks plugin (1.1-alpha17.235.354, latest as of writing). If this plugin is installed on TeamCity 2018.1.2 the UI bugs out completely. We uninstalled this plugin and the problem cleared up completely. Also, the plugin has now issue with TeamCity 2018.1.1.
If you are depending on this plugin, don’t upgrade to 2018.1.2 until the plugin has a new compatible version.