Format Rule Stopped when changing Index Key

Hi,
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.
appsheet1b|690x284

Do these images ilustrate the problem?

Thanks

What is the format rule expression?