The bug

The game output log sometimes just does not display exception stack traces.
Clicking repeatedly on arbitrary filter buttons (e.g. "fatal" even if there are no fatal log messages) might show the exception stack traces.

How to reproduce

1. In Minecraft use the following command

   /summon armor_stand ~ ~ ~ {CustomName:invalid-json}
   

   It should fail because the CustomName is not valid JSON

2. Look in the launcher log
   It only shows the warning text "Exception loading entity:", but the stack trace is missing. You can verify that the game logged the stack trace by looking at the log file.