I could not reproduce this so far. I used a search to filter out events which had
certain category tags on it. Then I used the list view to see all remaining events. I used the right-mouse pop-up menu and "...move to calendar" to move these events from the default calendar to a different calendar. I first tried to select multiple events via Shift-Click, but that would move only one item anyway.
Having finished all my moving clicking to different months and coming back, certain events were gone. I closed evolution, even via the "--force shutdown" command line, reopened it and the events were gone. I used a grep on the ics files to confirm that the data was indeed gone. Then I re-imported the data from a backup and tried to reproduce this but could not.