Can you take it apart again?
I'm looking at the 12t from my 11-28 spare cassette.

There is one tooth with a shark fin shape opposite 2 teeth on the other side that have the side carved out.
Laid flat on the desk, none of them are out of line to the outside.
The shark tooth one leans a tiny bit to the inside.
I can slip my fingernail under the shark tooth and the half width teeth, but the rest are flat against the desk.

I think you should take it apart, wipe it down and make sure no crud got between any of the spacers or cogs while it was being put together.