@Anmol12  What do you mean by rich_description field is not present on task table ? 

I am just doing dot walk to get parent record field value, in this case parent is holding HR Case record and rich_description exist there so it can fetch the value. 

Have you tried to reproduce this on your personal instance ?

If you are doing it on your org instance then try to check any existing BR is not contradicting here ? Also try to lower your BR order to see if it executes or not, add some logs in BR . 

Let me know if you have any further question for me.

Thanks,

Harsh