Hello, Could you provide a screenshot?

When you add the desired field, click 'Save,' leave the tab and then return, does the field appear blank?

“I’ve created another automation for a different action. After that, I tried the first one again, and it worked fine. I don’t understand what happened, but now everything is working well.”

That's great news, the most important thing is that the automation is now working as expected. Please don't hesitate to reach out if the issue reoccurs.