Is it permissible to feign eating and drinking during a voluntary fast to avoid showing off?

It is permissible to show one's fasting if there is an overriding benefit, such as alerting others to fast on virtuous days, or to be an example, or to thank Allah Almighty for the blessing of guidance. The general principle is that it is desirable to conceal acts of obedience unless there is a need to show them. What is blameworthy is performing an act of worship or showing it to seek a worldly benefit from people, such as their praise.