Hi Leyla,
It sounds like the 23" HP monitor is connected to one of the DisplayPort outputs on the dock (not the “HDMI 4K” port).
Please note that the two DisplayPort outputs on the dock work through USB graphics known as DisplayLink. With the DisplayLink driver, video data is compressed and sent to the dock as USB data (frame buffers), to be decoded and displayed on the monitor.
The HDMI 4K port on the other hand does not use the DisplayLink driver, but rather relies on USB-C Alt Mode video. USB-C ports optionally support the ability to output a video signal (powered through the native graphics processor in the computer) to drive a display.
However, the USB-C port on this computer does not support video as outlined in the manual.
I hope this helps clarify.
Mitchell