Except at very low speed, a bike is "steered" by leaning, not by using the handlebar to "steer". Your fork is not backwards. It is possible that what you are experiencing on one bike and not the other could be due to differences in the geometry between the two, if it is happening at lower speeds. Seems like you aren't confident in the prep by the mechanic. If for nothing but peace of mind, you might want to have a different shop with a reputable mechanic go over the bike.