In the background, is it a property (accessible via the document options and configurable according to the size) or only a text with the annotation "A4"?
Sometimes you change it unintentionally, so the text box is no longer affected by the document's ownership.
Is it by right-clicking on the sheet then "Edit the basemap", by clicking on the annotation "A4" (or other format) then linking to the property (see attached doc) and selecting the SWFormatsize property.
By changing the dimension, it must follow, is that the case?
@Coyote: Yes too, but I use the problem function here and everything works perfectly at home. Moreover, it's a function managed by SW directly so it shouldn't block...