Although this is a common criticism of utilitarianism in the Bentham traditition, it is not in fact how utilitarian ethics work as a whole. For example John Stuart Mill would say that whether or not an act in a specific case is ethical can be decided by whether or not it is in violation of a general rule, and whether or not the consequences of the general rule are positive or not. Taking clients funds and using them to go double or nothing would clearly lead to lack of trustworthiness and the consequences of this would be clearly negative in general so this is very clearly unethical according to Mill.[1]

[1] https://www.britannica.com/topic/utilitarianism-philosophy

Very few utilitarians or affective altruists (including me) consider that ethical.