I recently came across a scenario where the Retail POS suddenly started showing more than 2 decimal places. There are no changes in the Store or Retail configurations which could have triggered the sudden 3 decimal places to appear at the POS for the Amount fields.
I was aware that the currency plays an important part in the decimal and rounding places. After a while found that the Currency form's Rounding Rules, Prices field with 4 decimal places when changed will affect the prices in Retail POS. So after I reset it back to 0.0000 instead of 0.0010, the issue went away. After the change make sure to run the Currency job to sync the changes to POS, log off and log back in to see the new changes. The rounding should now have come back to 2 decimal places.