If you want to see what I see regarding this error, set up a fake account, single (doesn't matter), income of say, $60,000. Before you fill in the Rebate worksheet, check the amount of the Federal tax subtraction on the Oregon return. Now go to the (federal) Rebate worksheet and type in $1200 received for the 1st stimulus. Leave the 2nd stimulus blank. Go back to Oregon and check the Federal tax subtraction now. You'll see a decrease of $1200 there. Now return to the Rebate worksheet and type in $600 received for the 2nd stimulus. Return to Oregon and check the Federal tax subtraction again. NO CHANGE!! 

Ok, now delete what was typed on the Rebate worksheet and start over. This time, indicate that you have NOT received any rebates. In other words, fill in zero for each stimulus received. Go back to the Oregon return. You'll see that the Federal subtraction has been reduced by a FULL $1800.

Does that make sense? It does not.