Strange behavior: repository_dispatch pull request call actions from main branch

I use repository_dispatch for chatbot to manage actions by-demand in Pull Requests
Unforthunatley this actions executed from main branch only, not from PR brainch
There is my live sample describled with details:

I was created PR and change echo reply string from Pong from to Test Pong from
When I execute ping, I was received reply from main branch
After merge, I receive correct reply from main branch again