Is this normal? I find an answer here that this is normal but I do not understand. Can anyone add an explanation, please.
AFCI is Square D 20A HOM120CAFIC, Issue No. DP-3640 HACR Type
I went and looked up the manufacturer's page for your breaker. I noticed that they have a document on that page: Homeline™ Circuit Breaker and Combination Arc Fault Circuit Interrupter (CAFI). The important part of that PDF is on page 3.
Your breaker is a combination breaker, meaning that it does both AFCI and some GFCI (it's not a "true" GFCI in that you don't have a test button for a ground fault). You'll note the manual specifically says it handles Ground Faults including Ground to Neutral. When your neutral touches the ground (while the neutral is carrying a current) it creates a leak in the current because some of that current will go through the ground. If you look at the breaker without the panel cover on (be VERY careful if you do!) you'll note the neutral wire runs into the breaker and a separate "pig tail" runs to the neutral bar.
So GFCI part of your breaker is causing it to trip by detecting the grounded neutral. This is by design.
When you touch neutral to ground, two things are happening.
Many AFCI breakers have a design that is sensitive to ground faults, so a ground fault may trip those AFCIs. They are not rated for GFCI, however.