Just as fulfilling all "faraiz" is obligatory upon a Muslim, it is also obligatory to acquire all shariah knowledge relevant to a "farz" before attempting to fulfill that "farz".
Therefore, for a person living in an Islamic country where there is no barrier to gaining knowledge about deen, ignorance regarding the shariah rulings of a "farz" is not considered an excuse in Shariah. In light of these two fundamental principles, the answer to your question is that there is indeed a difference between ignorance and forgetfulness.
Ignorance refers to a lack of knowledge regarding the time for the end of suhoor, while forgetfulness refers to the situation in which an individual is aware of the time for the end of suhoor but forgets that they are fasting.
Therefore, if someone is unaware of the time for the end of suhoor and continues to eat even after the time for suhoor has ended, assuming there is still time, their fast will not be considered valid. On the other hand, if an individual is aware of the time for the end of suhoor and stops eating at that time, but forgets about their fast and eats or drinks something afterward, their fast will remain valid.