You did nothing wrong, the grass done let you down. In this case, it looks like it didn't take and didn't re-root properly. Does the area over the septic tank have very thin soil? If so, in the future, you'll simply have to water a little more to encourage as much rooting as you can.

From previous photos, you have a tri-mix, so feed and water as you should, ride it through the summer, and reseed in fall again. Bring it in, and treat it well, and plan to coddle it through summer 2021.