I have seen some odd things in tanks over the years where an individual fish had issues. Here are two such similar examples.
In my first tank I had a pair of Trichogaster lalius (Dwarf Gourami}. I was pretty much still a gross newbie then, The tank was a planted 45 gal. community. The male built a bubble nest and there were eggs. I watchrf how any time the female came into view of the male, he attacked her. After a day or two I was coming into the room where the tank was at the opposite end of the door. The male made a move on her and she bolted hard into the glass. This is a small fish and I heard the bang into the glass across the room. Not long after this she was dead.
Fast forward a number of years and I was trying to see a Discus spawn from beginning to when the kids ate slime off the parents. No matter which pair was in the tank, they laid eggs but they were never fertile. I switched males twice. The second time I was sold a male that laid eggs within a week- DOH. This was a 25 gal tank dedicated to a discus pair. It had plants in pots but no substrate. Finally, I gave up hope. I had parked a couple of adult zebra plecos in the tank with the discus. Over time I lost one discus (cause unkown) and the remaining fish was fine. Some weeks later I came into the bathroom where the tank was and I guess I spooked the fish. It swam hard into the glass. But it seemed OK right afterwards.
Not long after I noticed the discus was having difficulty in remaining upright. It seemed fine except for living face down not foward. Ignorant me thought it was swim bladder related and treated for that. It did not help, By this time the injured discus was the only fish in the tank as the zebras got a real home. I kept that discus alive for many months. I am not sure I was right to do this in hindsight, But eventually it died.
The point of these two stories is simple. Had I not actually witnessed the collisions into the glass but only seen the aftereffects, I would have had no idea what was wrong with either fish. The female gourami would just have been alive earlier and suddenly dead without a mark. At least the dixcus lived, albeit nose down. But, I would have made the same wrong guess about the problem and treated it the same way.
As far as I am concerned this hobby is a journey of learnin for those who are interested enough in everything. For some, their fish and tanks do fine almost all the time and none of this ever comes up. For others there are problems to solve when they only have very little experience or knowledge. At the risk of repeating myself, when faced with issues with our fish where we are not certain of what to do, most of us will do the best we can rather than watch fish suffer and die. I would rather make a mistake than do nothing most times. But that is me.
Sometime we are sure a fish will die and it recovers and still we do not know what was wrong or why it is better. Other times we are sure the fish will recover and yet it dies. I currently have 400+ fish in my Tanks (many because I spawn a few species of plecos). I do not tend to lose many fish now, save to age. As I back up in time the numbers were much greater as a % of the total. Most of us tend to improve our fish keeping skills with time.
I do not think we will ever know what is up with the tetra. That is how it goes somethimes. What is important is that the problem is limited and does not affect the other inhabitants of the tank. If it makes you feel better/safer, you can move it to a small H tank if you are able.