these two options have always been confusing to me with lots of overlap and similar UI.
it helps to realize that using either of those dumps you into an editor mode sub-menu of settings > window rules, which you can back out of by clicking on the < in the window header.
using the context menu just fills in the description and window class details for you vs. when go in thru the settings page, those are blank.
nothing seems to be saved unless you use + Add New… and Apply, then you can navigate back and see that it has been added to the settings page where you can now sort, edit, duplicate, or delete them.
beyond that cursory understanding i’ve never really been able to get these settings to bend to my will either. In fact, looking at it just now i realized there was a rule in place causing gthumb to not maximize properly due to a size rule, which i have now deleted.