Welcome back. I think you're correct, that it is damping off.
Once I've picked through my germination jars, and taken all the seedlings I want, I'm not as careful about those jars as I should be. In the image below, damping off has killed the seedlings in the upper half, and spared the lower half. I had left the lid resting loosely on the jar, while it was exposed to partial day direct sun. I believe the dead half was blocked from the sun by the lid, whereas the live half directly faced the window.
Usually, just adequate water, a thin sun shade (a single layer of Agribon AG-15), and direct sun exposure suppresses the fungus, and spares the remainder.
Bob