Praise be to Allah.
We do not know of any scholar who says that a bee sting or scorpion sting is one of the things that invalidate the fast. Bees and scorpions inject poison into the body of the one who is stung and he tries to avoid their sting, whereas in the case of all the things that invalidate the fast, it is stipulated that in order for the fast to be invalidated, the fasting person does them voluntarily.
But things that are done involuntarily do not invalidate the fast, as in the case of a person into whose mouth a fly flies and he swallows it – the fast is not invalidated by that.
There is no known difference of opinion among the contemporary scholars that injections to the skin or muscles do not invalidate the fast. The basic principle is that the fast remains valid unless there is proof that it has been invalidated.
These injections are not food or drink, and they do not take the place of food or drink, so if the injection is medicine, it does not invalidate the fast, so it is more likely that the sting of a scorpion or bee does not invalidate the fast.
For more, please see these answers: 38023 , 2299 , 124205 , and 95062 .
And Allah knows best.