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Based on BootstrapVue documentation, Sorting/filtering/paginating the table will clear the active selection. The row-selected event will be emitted with an empty array ([]) if needed.
My suggestion is to store the selected rows to another variable (eg: selectedList: []) on row-selected, so that it will not emitted with an empty array when pagination trigerred. Then, append the selectedList on refreshed Event, so that when you go to next page or ack to previous page, selectedList variable will hold all rows you’ve selected.
Source:stackexchange.com