Why is this thread still alive?
If you under-report your income and pay too low estimated taxes, when tax time comes around and your true (higher) income is stated on your tax return, you're going to have to pay the difference between your full tax liability and the amount of taxes you actually paid during the year.
If you under-report your income and receive ACA subsidies which are too high, when tax time comes around and your true (higher) income is stated on your tax return, your going to have to pay back the difference between the tax subsidy you actually received during the year and the (lower) tax subsidy you should have received during the year.
In both cases, the effect is that you end up paying the correct net tax and receiving the correct net ACA subsidy you should have paid/received all year long.
Now, why is this completely fair result a problem?