A lightbulb went off after reading this. I wonder if it's entirely dependent on the type of tank they spent their formative weeks in. The fry left behind spend a lot of time picking at the algae encrusted wood. They eat when I feed them too. BUT...the previous batch from this pair, also raised in this tank, I had a lot of issues with when I moved them to traditional bare bottom tanks. It occurred to me that they were used to grazing, and probably a lot more green in their diet, then I switched them over to a bare tank and a schedule with more protein than they were used to.
I couldn't figure it out because I'd "stolen" a group of surviving wrigglers from another pair who always chose the filter intake. Those grew up in various bare bottom containers so ate only when I feed them rather than grazing. They also didn't have a rash of inexplicable losses.
My mind is aglow with trancient nodes of thought, careening through a cosmic vapor of invention - Hedley Lamar