That would open an interesting can of worms.
1. It is common under many (most?) legal systems for those harmed by another's negligence to have a duty to mitigate damages.
For example, if I'm burning something on my property and an ember from my fire sets something on fire on your property, which you see happen, and you could put the fire out before it causes much damage easily with your garden hose but instead just watch it burn, I'm probably only going to by liable for that portion of the damage that would have happened had you used the hose.
A few governments could make a case for full reparations, such as South Korea, Vietnam, and a few others. The US, most of Europe, and much of South America, on the other hand, would have a hard time arguing that most of their harm, especially after the first wave, was not due to their own poor handling of the pandemic.
2. If the release was accidental but not negligent, the "Act of God" principle may apply.
That principle is that some failures are just expected in the course of some activity that is commonly done, and if the failure is not caused by negligence it is written off as an act of God, and it is up the victim to deal with.
For example, my neighbor has several 100 ft (30 m) tall trees that when I look at them from my back door I have to look up at a 60 degree angle to see their tops, which means that they are much closer than 100 ft (30 m) from my house.. If one of those trees fell over in my direction the damage to my house (and me depending on where I was at the time) would be extensive.
Whether my neighbor would pay for that damage or I would depends on the health of the tree. If the tree was dying or dead then the neighbor would be liable. They are supposed to watch for those things and have such trees safely removed. If the tree was healthy and just caught a bad break with the wind then I have to deal with it. An act of God brought it down and it is my problem.
Several countries operate labs working with potential pandemic causing viruses. Accidents have happened before, and will happen again, but they all continue operating those labs. They know at the time they build these labs that these accidents happen even in the best run labs, they calculate about how often they will happen, and decide that the benefits (often military benefits) are worth it in the long run.
I could see a country that has one of those accidents argue that at least in relation to other countries that also run such labs that their release was one of those "normal" accidents, not a negligent accident, and so each country that itself runs such labs is responsible for handling its own damages.
3. Some of the countries that would most like to receive reparations also do or have done things that have caused widespread harm outside their countries, which one might argue should lead to reparations. It may be hard to find a way to set the boundary on what should or should not require reparations that has COVID on the "yes" side and the things that they have done on the "no" side. They may decide it is better to keep the lid firmly on that Pandora's box.