I have a scrum board. In the current sprint there there are ~90 completed stories and 8 incomplete ones.
When I invoked the 'Complete Sprint' action it prompted me what to do with the incomplete ones. I chose 'move to next sprint' and chose the specific sprint.
Out of the 8 stories, 5 stories were moved to the selected sprint, as it should be, but 3 stories were moved to the backlog.
AFAIK, the 'complete sprint' action does not care about the columns in the board - it decides which action to take on each story based solely on the Status Category of each story.
Any idea why the different behavior?