User defined print size does not have the option to save

In Libre writer v5.0.4.2, the option to save a user defined print size is grayed out. This has been the case for several prior versions. It used to work, but hasn’t in several versions. Am I doing something wrong or has the bug lasted this long?
I change the print size to a user defined value, but the “Save” button continues to be grayed out.

Thanks.