Hi Nick, thanks for looking at this.
I tried again and got the same behaviour:
1: Create the task in this case: "Test"
2:.Add a deadline with reminder two days before by clicking on Due Date
3: Add a monthly repeat
4: This is the final result after putting in the repeat.
5: Then I tick the checkbox at the beginning of the highlighted task and this is the result:
As you can see above, the due date is now Sat, 16 November not 15 December.
This was all done on the web in Firefox 24.0