@Jenna_Avery thanks for asking! Actually, the recurring task due dates are setup this way by design. As you can see in this blog post,
In addition to the normal schedules like daily, weekly, monthly, and yearly, you can also choose “periodically” for tasks that should repeat based on the last time the task was completed.
For example, I like to send the team a summary of customer feedback every week. I can set the due date to Monday, and have it repeat 7 days after completion. If I have a busy week and don’t write the summary until Thursday, when I mark it complete it will create the next task for next Thursday. This is really nice because my tasks don’t get crowded close together, and get scheduled for just when I want them.
Also, this Asana Guide article on recurring tasks indicates that the interval you’ve set for your tasks’s recurrence will determine the new due date.
That said, I can understand how this can become challenging sometimes. I have had a similar experience. 
If you’d like people to stick with the same due day of the week, I suggest that you ask them to reset the due day of the week after they complete the task late. So, if they complete the task on Tuesday but it was due on Monday, ask that they change the due day back to Monday when they complete the task. This ask is something I’d categorize as Asana etiquette or a trust policy of sorts.