This seems to be a sticky situation for the beginning of the year, and although it is a pressure point for President Trump, it seems to me that it causes undue complications during the height of the tax season.
For instance, the IRS requires all previous year taxes to be filed by April 15th, but taxes can be filed as soon as the W-2 (or equivalent) is given. Due to the individual mandate, taxes are being taken as a penalty.
If the ACA individual mandate is repealed, how will that affect someone who has already filed (and paid) on their taxes?