For R2018b and later versions, the default value of the documentation reference has been changed from "local" to "Web" so that the latest documentation can always be viewed.
For "Web", you need to connect to the internet and log in to your MathWorks account.
(In the environment without an Internet connection, the default value is "local".)
If you need the proxy settings for the internet connection in your environment, select the "Preferences" icon on your MATLAB "Home" tab. Then click on MATLAB and select "Web".
Please refer to the following link for more information about the web browsers and MATLAB:
Also, please make sure you are logged into your MathWorks account. In some environments, a known problem has been reported that the help document is not displayed correctly even if the correct proxy settings are made.
This problem is fixed in R2018b Update 3. For details, please see the following URL:
Also, it is possible to set the reference of the help document to "local" and refer to the document in the local folder as in R2018a and earlier.
Select the "Preferences" icon in the "Home" tab and change the settings from [MATLAB] -> [Help].