Not sure if this is a bug or works as intended:
Attack rolls when attacking with the off hand do not add str/dex modifier to the attack roll.
However, str/dex are added to the damage roll, even without the two-weapon fighting style.
When you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can use a bonus action to attack with a different light melee weapon that you're holding in the other hand. You don't add your ability modifier to the damage of the bonus attack, unless that modifier is negative.
