[REG 10.0.2 -> 11.0.0] Unclear counting of tasks

1. Open the attached task file in Creator.
   It contains 1100 tasks: 515 warnings, 484 errors and 101 other tasks.
2. Check the numbers Creator shows on its "Issues" view button.
   It displays "853" when warnings are included and "410" when only errors should be shown. It isn't clear where these numbers come from.

Creator 10 displays "999" (= 515 + 484) and "484", respectively.

Found by Squish test tst_tasks_handling.