null variable in Email when Approval Flow is reassigned

I'm implementing a Flow approval process. and when doing reassign action the user is getting this email: 

the email is built-in. I haven't created it. I can't find where this null is coming from. and there's no customization for this template. I mean I can't even find this template from setup.

And the reassign action itself doesn't contain much of a customization: 

as you can see I filled all options, even comment just to test, but still user's getting null.

Does anybody know what this null can be?