Error List: "Build + IntelliSense" pane does not contain an error shown in the "Build" pane.

The following steps demonstrate a problem with Visual Studio v17.10.5.

• Open the .sln file.
• Open Program.cs.
  (The problem doesn't occur if Program.cs isn't open in a Visual Studio editor window before it's compiled.)
• Build the solution.
  The following warning will be displayed in the Output window:
  ... Program.cs(9,17,9,21): warning CS8602: Dereference of a possibly null reference.
• Check Error List; The CS8602 warning appears in the "Build" pane" but the warning does not appear in the "Build + IntelliSense" pane!
  (If the warning didn't appear in the "Build" pane then select another pane and then go back to the "Build" pane - That problem was reported here: https://developercommunity.visualstudio.com/t/Build-error-missing-from-Error-List/10705983.)