The tax cut was fully offset by money from general revenue so that the trust fund was unaffected by the tax cut.
Therefore, Social Security is now dependent on a certain amount of money out of the normal budget. Before, the Social Security Trust Fund was entirely separate. It never needed general revenue money and therefore didn't contribute to the general revenue deficits. Now, to a small extent, it does.
...it's a one time arrangement. You're implying that replacing the funds changes the fact that Social Security by law is a self-funding entity. It's on the government to replace the funds to satisfy the arrangement. It has nothing to do with Social Security's structure.