I cannot find the reason why it does not work on your website.
I had copied your shortcode and use it on my test site.
I am able to get the auto-generated excerpt to work properly on my test site.
As a workaround, Please use the crafted excerpt setting ( Zajawka setting )
I tried adding the sentence “This is a crafted excerpt” into your latest post and it worked.
At the moment please use this workaround.