External display works on 30Hz
I had the same issue for a long time. At first, 60Hz worked. Then something happened and I'd only get 30Hz on my 4K Lenovo ThinkVision 27".

I stumbled upon this article and simply went to Display settings and hit "Resolution: Default for display". Immediately the display switched to 60Hz. Then I could reapply my Scaled text size setting and 60Hz remained.

What an unsatisfying fix!