Last year Madhu didn't tell RK she was fasting, she was hiding it for some reason known to her only, it can't be because he doesn't believe in the ritual because she didn't know that when she started to do the fast, she only found out afterwards when he figured out the mehndi was for karwa chauth and shouted at her badly and told her to go out of the make up van/coach of his on the set. Now this year he accepted to do the karwa chauth with her before and even made up for it by creating a date replaying karva chauth the way it should have been done for them minus the argument and the fainting Madhu did last year. So is it some extra ritual Madhu does where she doesn't tell her husband she is doing the fast or perhaps out of modesty not to tell anyone and do the fast without making a big issue where everyone has to be mindful she is fasting, if that's the case not telling people in fact is making it more of an issue because surely with the scene where she has to act eating dinner will cause her to be pretending to eat and then they notice and stop the shot another day lost without shooting and cause them to go behind schedule, rather she told them before hand and what would have happened in the planning of the shot they would have made sure to postpone the dinner sequence to another day and shoot something else, better not to hide it because of this scenario how it will play out.