It looks like IntelliJ for some reason uses the Metal LAF (or similar) for the title bar when Material Theme UI is activated since the new 2021.2 EAP 6 released today.
Also, since 2021.2 EAP 2 or something, the bar under the title bar just mirrors some part of the screen (also seen on the screenshot
Steps to reproduce (if needed)
Install IntelliJ 2021.2 EAP 6 and the newest version of Material Theme UI
Start it and see the title bar change
Deactivate it again and the title bar is back to default (without bugs)
Summary
It looks like IntelliJ for some reason uses the Metal LAF (or similar) for the title bar when Material Theme UI is activated since the new 2021.2 EAP 6 released today.
Also, since 2021.2 EAP 2 or something, the bar under the title bar just mirrors some part of the screen (also seen on the screenshot
Steps to reproduce (if needed)
Environment
It should contain:
Screenshot