It looks that in newer versions of Matlab, some HMI (Human Machine Interface, e.g., Diagnostic Viewer, Live Scripting) functions are realized by means of local network port communication.
However, this approach seems unreliable. From year 2014 to 2020, many users reported the blank Diagnostic Viewer issue in various Matlab versions. Unfortunately, the bug may still not be effectively resolved.
Recently, Augusto Sérgio SIlva de Paula posted a workaround for this issue:
Use this command to delete the Diagnostic Viewer’s instance, and run the simulation again, the Diagnostic Viewer will work.
Although this is not a solution for this issue, but it avoids restarting Matlab when facing this bug. Thus, it is a very good trick, thanks for Augusto Sérgio SIlva de Paula.