Shaykh Bin Baz was asked:
May Allah be good to you Shaykh. Whoever returns to disobedience after Ramadan, is that a sign that his actions in Ramadan are not accepted?
“It is feared for him because this is a sign of leniency and that [his] repentance was not truthful. It is necessary for the believer, if Allah favors him to complete the month of Ramadan, to be steadfast and to continue upon good. He should be cautious of evil and returning to it for returning to evil is dangerous. It is a cause for his good deeds to be invalidated and not be accepted. There is no power nor strength except with Allah. We ask Allah for safety”.
[Sharh Wathaaif Ramadan pg. 162]
Faisal Ibn Abdul Qaadir Ibn Hassan