"I took funds out of my 529 account in August 2018 for fall semester and paid the university that same month."
That's not an issue for your 2018 taxes. Provided the money was used to pay qualified education expenses, that "cancels out" the taxability of that withdrawal.
"I then took out more 529 funds in December of 2018 but did not pay the university until January 2019. "
That could be a problem. However, 529 funds can also be used to pay for room and board, provided that room and board was in direct support of the education. So do you have room and board expenses that you paid in 2018, that you can claim against all 529 funds that were not used to pay qualified education expenses with? If so, then your issue is now a non-issue.
