Is this an accumulation of lasting damage to assorted organs (including brain) and the direct and higher order effects of that? Or is there some ongoing infection to be combatted?
https://www.youtube.com/watch?v=278vwGkFXRM
There's many more of these types of videos by the NIH and other large groups too.
There are also reviews here: https://www.nature.com/articles/s41579-022-00846-2
Short answer is...we somewhat have hypotheses around the cause. We don't know much about the lasting damage yet.
Many folks who have long covid do not have any organ damage from acute infection. But the big question is if a persistent infection causes damage over the duration.
There is also some evidence that some of the significant mutations / new strains arose not from "natural selection" via transmission, but from prolonged mutation in a single host.
I have read a lot of research on this topic and I have not seen this suggestion, rather I have only seen the neutral (that it hasn't mutated from persistence/potential replication in a host).
Do you have a link to a study that suggests this?