Oxidation on the underside iPhone XS Max

Hey, guys...

it has spearing a brown spot on the underside of my iPhone. It’s like an oxidation. I had this problem, in the same place, 1 month ago. The apple support didn’t recognize it as a problem and I had many problem to change it. I got it!

But now, the same spot is appearing again. Anyone else with this problem.

ps. It’s close to lightning entrance.

since now,

thanks everyone!