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.
So yes, I do believe there's a bug in Two-Weapon fighting.
I also noticed that I often have a higher chance to hit with my main hand than off-hand, which I find odd, since the rule states nothing about not adding modifiers on your to hit roll.
You are also able to use off-hand attack twice with the Thiefs two bonus actions, which also break the rules.