Hi Enrico,
Sorry that you are running into this issue.
I can reproduce the problem. I believe the presence of an Extent Indicator element in the Map Frame is interfering with subsequent property changes (ex: clicking the up/down arrow of the Offset control for a label component) made on a Grid element within the same Map Frame. If an Extent Indicator is not present I am able to make subsequent changes and see them reflected in the Layout view.
For a work around please try removing the Extent Indicator, making the changes you want to the Grid, and then when you are done with the Grid, add the Extent Indicator back to the Map Frame.
Can you contact Esri Support and report this issue? Having an issue reported through support helps us prioritize our work. They can be contacted at https://support.esri.com/en/contact-tech-support.
Thanks for posting this.
Tom