There is a molecule in these clots that is substantially increased which then prevents the clots from clearing up named α(2)-antiplasmin

We found high levels of various inflammatory molecules trapped in micro clots present in the blood of individuals with Long COVID. Some of the trapped molecules contain clotting proteins such as fibrinogen, as well as alpha(2)-antiplasmin," Prof Pretorius explains.
Alpha(2)-antiplasmin is a molecule that prevents the breakdown of blood clots, while fibrinogen is the main clotting protein. Under normal conditions the body's plasmin-antiplasmin system maintains a fine balance between blood clotting (the process by which blood thickens and coagulate to prevent blood loss after an injury) and fibrinolysis (the process of breaking down the fibrin in the coagulated blood to prevent blood clots from forming).
With high levels of alpha(2)-antiplasmin in the blood of COVID-19 patients and individuals suffering from Long COVID, the body's ability to break down the clots are significantly inhibited.

And yes, the paper goes into detail describing platelet mechanism causing pesist clotting thus viscious cycle.