Joining Two Models with Rounded Corners Always Ended with a Jagged Rounded Corners

I want to combine these two models:

From the top side:

From the side:

After I joined them:

The top side is okay, it is perfect:

But from the side the rounded corners become jagged like this:

I am pretty sure, I have aligned the model with Workplane and pressed D to drop the model. So... what I should I do to join both models properly?