It appears that TurboTax is producing a nonsensical result, but it's because you have made an error that is triggering this behavior. Box 5 of the Form 1099-R says that the distribution includes $10,008 of after-tax contributions, so entering $10,000 when asked how much of after-tax money was included in the distribution is an incorrect entry; the correct entry is $10,008. An entry here different from what is in box 5 only makes sense if box 5 is blank because the payer does not know the correct amount, has also left box 2a blank and has marked box 2b Taxable amount not determined.
It seems that making the $10,000 entry confusing TurboTax because if the $10,000 entry was correct because boxes 5 and 2a were blank, the appropriate result would be $20 taxable. With your incorrect entry of $10,000 TurboTax is doing this calculation, but because box 2a is not blank when it's expected to be, TurboTax is also adding the $12 from box 2a. With the entry of conflicting information, I don't think TurboTax can be faulted for producing a nonsensical result.
The reason that your distribution included $10,008 in box 5 and not just the $10,000 that you thought was correct is something that you would have to work out from your records and other information from the plan.
