That seems to me to be a direct result of too many actors involved in the creation of the Deutschlandticket actually ebing against this ticket, and so those in favor tried to appease them and created this absolut disaster of a booking process. There is no other reason for this otherwise, and it would be trivial to sort this out by e.g. having a validity period of 31 days, or always going until the end of the month and costing only a percentage. But that would be too easy.