Format Rule Stopped when changing Index Key

Probably I am doing something wrong that I can´t figured out.
I have a text column with different values everything works ok until i change the rownumber key to another column key,
Thanks in advanced

In what way aren’t things working?

Thanks for your response,
With Key Column marked to Codigo de Tarea format rule does not work:


With Index Key marked to _Rownumber format rule works ok.

Do these images ilustrate the problem?


What is the format rule expression?