Since a Roth contribution doesn't lower taxable income, I am not sure why the Marketplace would adjust the advanced credit for it. This is why I assumed they misunderstood it to be a traditional deductible contribution.
The premium tax credit is determined based solely off of the Modified Adjusted Gross Income (MAGI) on the return, and the information on the 1095-A. Roth IRA contributions don't affect the MAGI, so they aren't in the calculation at all.
I can't be sure what happened at the Marketplace but making a Roth IRA contribution doesn't increase the premium tax credits. You can see the credit calculation on Form 8962.
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