This is somewhat ambiguous (at least to me). The 529 distribution must be taken in the same year that the corresponding qualified expense is actually paid, not necessarily in the same year that the qualified expense was billed.
Example: the college sends and the student receives in December 2019 a bill for tuition, room and board for the spring semester that begins in January 2020. If the bill is paid in December, the corresponding 529 distribution to fully or partially pay/reimburse these qualified expenses must be taken in 2019. If the bill is paid in January, the corresponding 529 distribution to fully or partially pay/reimburse these qualified expenses must be taken in 2020.