It is tricky, though. In your case, yeah, it would have been perfectly valid to insert the completion for all the cursors, but in another case it might not be good. There is no guarantee that all the cursors are in the same context.
The completion happened to be suggested for the last cursor, so that is where it was processed. There are really only two solutions (1) disable auto-complete when we have multiple cursors or (2) do a lot of work to do the same insertion for all cursors, potentially creating a real mess in the process.