I found this:

https://www.irs.gov/newsroom/medical-loss-ratio-mlr-faqs

"E. Effect of MLR Rebate on Taxpayers Who Claimed a Premium Tax Credit

Q15. Hanna enrolled in a qualified health plan through a Health Insurance Marketplace for coverage in 2018.  She did not deduct any of the premiums for her coverage on her 2018 Form 1040.  When filing her 2018 federal income tax return, Hanna correctly completed Form 8962, Premium Tax Credit (PTC). Based on her Form 8962, Hanna was entitled to, and claimed, a premium tax credit for the coverage. In July of 2019, Hanna received an MLR rebate based on her enrollment in 2018.  What are the federal income tax consequences to Hanna for the receipt of the MLR rebate?

A15. Because Hanna received her rebate in 2019, Hanna is not required to amend her 2018 tax return to report the rebate.  Also, because the rebate Hanna received in 2019 is merely an adjustment to the premiums for her 2018 insurance and Hanna did not deduct the premium payments on her 2018 Form 1040, she is not required to include the rebate in her 2019 income.  The Treasury Department and the IRS are considering the issuance of guidance applicable to future tax years that will address whether a taxpayer must increase his or her tax liability for the year of the receipt of the MLR rebate to the extent the taxpayer was allowed a premium tax credit for the portion of the taxpayer’s prior-year premium that was refunded."

You should read all the scenarios in that article. Example:

The subsidy is related to their income, not their premium. So, if a taxable rebate puts you above the 400%, then you would need to repay the subsidy for that year your income is too high.