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I think there may be some confusion about how how Reading List and marked books work exactly.
Calibre has a mechanism for temporarily 'marking' books. That's the pushpin icon next to the row number. It is not a column as such, and there's only one set of 'marks' available, but it is something you can search on. The most basic search is to search for marked:true. (Marks can also be named, which is a more advanced topic.)
When you 'View List' in Reading List, the PI marks each book in the list, then automatically sets the search to show only those marked books. It is not something that is saved as a column, it is only available until something else uses marks. Several other plugins also use marks and will reset all marks when used.
Also, if you clear a list while it's still being 'viewed', RL will clear those marks and now your search is empty. It detects if the list is being viewed by looking at the current search string for 'marked:reading_list_...'
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