Why'd I get an additional bill from the IRS?

No one at TurboTax knows.  The IRS does not provide information or feedback to TurboTax after your return is accepted.  What does the letter from the IRS say?  Does it have a CP number on it or a letter number?

Did they send you a bill for a penalty or interest ?   Read the entire notice carefully.

You are asking from TT Live, for which you are paying extra:

https://ttlc.intuit.com/questions/4124827-how-do-i-connect-with-a-tax-expert-in-turbotax-live

Live help is available M-F.