Dynamic Page Size appearing in Desktop

Hi,

I'm not sure how this happened but in a PBI file i'm using on the desktop I am seeing page size "dynamic". I have the March 2020 version installed and it is only affecting one page. If I add another page the dynamic option does not appear. If I duplicate the page it is there. If I change the page size to any of the standard formats such as 16:9 then "dynamic" disappears. I have a background image set to fit that is in 16:9 format but I don't know how the page size changed to Dynamic. I've also noticed with Dynamic selected that the "Page View" in the ribbon is greyed out.

Any ideas how it sets itself like that as I find that it's actually quite good and my visuals re-size nicely on the page and would like to use it in other files.

Thank you!