Top falls of seedling?

A question from a fellow grower:

seedlings from time to time once the first leaf appear, the tops fall off as if someone cut right under those first leaf, leaving only stem.
indoors, Philippines so can be humid and warm with fan.
Always same, new seedlings and maybe up to half of plants.
Again same place, one day ok and next the tops are laying next to stem.

Some bug, moth, or fly is chewing through your stems.