"Fill in Individual Fields" mode

Add a Hostname, say "a.b1.net"

Change the port number to a non-default port, say "33333".

Choose Kerberos Authentication

Add Principal, of "test@ANYCO.COM"

Add Service Name of "mongodb"

Choose Read Preference of "Primary Preferred"

Now, generate connection string by clicking "Paste connection string"

mongodb://test%40ANYCO.COM:null@a.b1.net:27017,a.b2.net:27017,a.b3.net:33333/test?gssapiServiceName=mongodb&authMechanism=GSSAPI&readPreference=primaryPreferred&authSource=%24external&appname=MongoDB%20Compass&ssl=false&authSource=$external

The port number is inconsistent. Both "33333" and "27017" are displayed

Click "Fill in connection fields individually". Notice the user-entered port number "33333" did not persist. "27017" is the displayed port number.

Click "Paste connection string" and change the port number to "1111".

Click "Fill in connection fields individually". Notice the user-entered port number "1111" did not persist. The original "33333" is displayed.