I would guess you're bunching CTCs on one spouse and the other spouse has income below the safe harbor for repayment?

Since the IRS never published guidance to stop this, seems like a valid tax avoidance strategy to me.

IRS released Fact Sheet guidance:

https://www.irs.gov/pub/newsroom/fs-2022-06.pdf

I see they issued 2022-07, as well.

Here are IRS resources, FAQ, and guidance:
https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-f...

https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021

Repayment protection (in Sched 8812):
https://www.irs.gov/instructions/i1040s8#en_US_2021_publink100077789