The appearance of the page grid in an object library window depends on whether an edition page is currently open on the Pages main tab or not, and which title is currently selected.

If an edition page is open (recommended) when you create or open a library object

If an edition page is open when you create or open a library object, the grid and the page dimensions of the library object is picked from the page geometry.

Please note that if the edition contains the pages with different masters grid and size, the currently open page may be unsuitable for the object. In such case, close the library object, open a page with a suitable geometry and then reopen the library object.

If no page is open (not recommended) when you create or open a library object

In that case, Fred uses the title's default geometry preset to draw the grid and to set the page dimensions of the library window:

The default geometry for the title is the one that has the Default check-box ticked (Action > Edit Object):

Please note that the default geometry may be unsuitable for the object.