A Vin of 12V with a Vout of 12V requires a duty cycle of 100%. However duty cycle is limited and for a certain output voltage there is a certain Vin range you can go for. For example for a 12V output with MAXM17544 the possible Vin range is 18.5V to 42V. Similarly for a 12V output with MAXM17546 the possible Vin range is 18V to 42V. Therefore, it is not possible to generate 12V out with a 12V in for these modules because of duty cycle restrictions.