Loop request with different data not working for params

Loop request with different data not working for params. I need to remove group from 100 users.
If I put username directly to param, its working. If I use {{username}} and json file with users, its not working, I get 400 - Returned if user requested an empty group name.

I have this simple set up:

Preview from data file is okay.

After I run collection, I get 400:

What in the world could be problem? I have same thing for create versions:

and this is working without any problems.

When username is written directly to params, its working. When its from file, I’m getting error from group name, which doesn’t make any sense, because group is there and its not changing. I also tried to write group name to json file, but I get same result.
docu: Jira 8.13.13

Any idea why its not working? thank you