>>When 'Type of result' is set to 'Only the first message of the thread', and From is used, only threads originated by From user are shown. The selection should not affect what threads are selected only what is shown.
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>Can you tell me more about it? I am having difficulty to understand what I need to look for.

Go to calendar, change forum to Universal Thread, put your name into From and click search. You'll get a few messages in the result.
Now, change 'Type of result' to 'Only the first message of the thread' and click search. How many results do you get?