The calendar on the connect website allows you to drag scheduled workouts from one day to another. You then receive a reminder about the workout in the connect mobile app and on your watch when starting an activity.
This all works fine when adding a workout to the calendar. But if you do the workout today, then drag the reminder to next week, you won't get any notification about it next week.
It seems to be that the reminder is marked as already done and that mark is not reset when moving the reminder to a future date which causes the watch and mobile app to ignore it. So either the rescheduled reminder needs to be marked as new again, or dragging a "done" reminder needs to be disabled on the calendar.