How long have you had these neons and what are your current water parameters?
Neons tend to be mass bred/produced now and aren’t always the healthiest when they arrive in the pet stores. So if they’re new, they may just unfortunately be dying and then your other fish may eat the bodies before you find it.
Similarly, if water parameters aren’t great they could be dying from that and same end result.
I doubt your pleco is catching and eating any neons. If any of your fish are likely culprits, I’d think its the eels, esp since I believe they’re nocturnal hunters? But I’ve never kept eels myself and don’t have much knowledge regarding that.
Edit: I just went to see your other post and it appears the other “eel” is a bichir. If it is indeed a fish eating your neons, that’s the likeliest culprit