There are some issues with Dual Wielding calculation.
Here is what I noticed:
- Off-hand weapon get ability modifier to damage, even without the Two-Weapon Fighting style, making that fighting style totally useless.
- Off-hand weapon doesn't get ability modifier to hit chance, resulting in very low hit chance.
- Dual Wielder feat doesn't grant the +1 AC, it's not just a UI issue, I made some tests, the extra AC is not added.
- Abilities or spells like Hunter's Mark should add extra damage on every attack made against the target, but it does only on the primary attack damage.
According to PHB rules, off-hand attack get same hit chance as primary one, but get ability modifier on damage only if you have the Two-Weapon Fighting style. I don't know if the lower hit chance is made on purpose, if it is, it's a really bad idea, TWF is already worst than GWF so you should not nerf it.