Hi @Remco! @Bastien is right, as it stands, setting “MT” to repeat every month won’t automatically update “MT” subtasks due date; this would have to be done manually I’m afraid.
I’ve gone ahead and merged your post with Recurring tasks and subtask dates as it covers the same topic. There are also some workaround available here: Expense Reimbursement Reoccurring Task 174
Hope this helps!