I have several maintenance tasks with "repeat after X" frequencies. Occasionally I do one of these tasks before the next due date. When I click "complete", the next iteration of the task has a due date of X after the original due date, not X after the date that I click it.

For example: I have a task "wash the sofa cover" that repeats every three months. However, due to a spill, I had to wash the sofa cover a month early. So I complete the task, and now the next occasion is due four months from now, rather than the three that I'd expect.

On overdue tasks, the reset works as expected; the new task is X after the day I finally clicked "complete", not X after the original due date. It's only "underdue" tasks where this problem happens.