Include pages in list of pages keeps getting checked automatically

I have a peculiar problem since this morning. I have the plugin Exclude Pages From Navigation to exclude some static pages that I need on the site from appearing on the menu. This worked fine till this morning and suddenly, the menu was overwhelmed by all these links.
I painstakingly unchecked the checkbox include in list of pages for each and every page, to no avail. As soon as I do this for the second page, the first one is back to the checked state almost as if it has a life of its own.
After a few attempts, I have finally moved them as a sub-menu of one of the menu items but that is not my aim and I am sure there is something weird going on that I am not able to put my finger on.
I have already tried deactivating and reactivating the plugin. It works fine on another website of mine.
TIA for your hel