Unfortunately for you, Arena handled that correctly. No player is given priority in the cleanup step, which is where discarding to hand size happens, unless a triggered ability triggered or a state based action was perfomed in that cleanup step. Discarding to hand size usually causes neither, and the ability to cast for mayhem doesn't qualify. (Note, that MADNESS would cause a trigger and thus a round of priority followed by another cleanup step.)

514.3. Normally, no player receives priority during the cleanup step, so no spells can be cast and no
abilities can be activated. However, this rule is subject to the following exception:

514.3a At this point, the game checks to see if any state-based actions would be performed and/or
any triggered abilities are waiting to be put onto the stack (including those that trigger “at the
beginning of the next cleanup step”). If so, those state-based actions are performed, then those
triggered abilities are put on the stack, then the active player gets priority. Players may cast
spells and activate abilities. Once the stack is empty and all players pass in succession, another
cleanup step begins.

unfortunately i was counting on it to to win and not let my opponent have lethal and ended up losing|