When doing a 401k to Roth 401k conversion, what month does is lump sum become part of your MAGI do determine Healthcare APTC or Medicaid? Is it January or December of the year conversion ?
When you fill out a Marketplace application, you’ll need to estimate what your household income is likely to be for the year.
For Medicaid purposes, you need to ask your state's Medicaid authorities that.
MAGI is calculated when you prepare your tax return. What you are concerned about instead is changes to the income that you estimated on your insurance application. You are to report changes in your estimated income on your insurance application as soon as possible after the estimate changes. If not already included in your original estimate of your annual income, you would need to report the update to your estimate upon doing the In-plan Roth Rollover since that income would be locked in at that point (absent the inadvertent rollover of some amount that was ineligible for rollover).