I can only offer unfelpful info based on the vid. Your angel reminds me of an experience a couple of years ago with imported Altums. It was a batch from the Rio Atabapo which required great effort to get to an importer in Texas. I realize that wild Altums and tank raised angels are worlds apart in many respects, but it was the manner of deaths that was not so different. The disease showed no external symptoms. No marks, no visible parasites or worms, no columnaris etc. Externally the fish remained looking normal right up to when they died.
The initial sign was a fish would become withdrawn from the group. It would tend to hang near the bottom for some time. Then it began to hang near the surface near filter flow and finally it went vertical with its mouth up. At this point it would be quite lethargic and soon after would die. These fish went into tanks tanks of several angel people on the same forum. In the end out of well over 100 fish sent out, one or two survived,
We all tried a variety of medications to no avail. We never got a handle on what it was. Presumably the fish were treated for internal parasites by the importer prior to the fish leaving their facility. I tried antibiotics on my 22 fish with no luck, they were all dead in under 3 weeks of arriving. I wish I had something better to offer.