I am not sure this question is answerable using physics, but it is answerable in magic. The answer is, 'Whatever you want to happen'. In allowing matter and antimatter to coexist, you have gone so far beyond physics as to make physics irrelevant.
We have no idea how antimatter would react to matter if they were not allowed to destroy each other. There is no valid reference in physics as to how the curse could have been implemented. How much energy would it have taken? Where did that energy come from? Because of this wizardry, could matter and antimatter atoms intermingle and intermix freely, or would it require some sort of energy barrier between them?
If this energy barrier collapsed, how much energy would it release?
We know, for instance, that fusion takes a lot of energy to initiate. But once initiated, it releases more energy than it took to initiate. I suspect that, whatever this curse, the removal would initiate some form of nuclear reaction, releasing far more energy than just the matter-antimatter reaction.
Physics tells us that the main radiation from such a process of matter-antimatter annihilation is gamma radiation, with great penetrating power. This would not be a simple 'explosion'. One can not, for instance, compare a nuclear explosion to that of an equivalent amount of TNT.
Antimatter is formed on earth during severe electrical storms. A lot of it. It is neutralized almost as soon as it is formed. But if it were not neutralized? How would it effect our ionosphere? Our magnetosphere? The ozone layer? Communications? It would change what we know about physics, and in reverting back to traditional physics, the implications are just as unknowable as the implications of making the curse in the first place.
To say that everything would be 'business as usual', except that antimatter is not annihilated by matter, simply puts physics aside. There WOULD be many, many other implications.
That last point is the crutch of any answer to the question. By what reason, except assumptions and wizardry, would antimatter atoms remain in the star? What would the theoretical physics be that explains this? How much energy would be required, and in what form?
This is important, because we do not know how much energy would be required to dispel the magic curse. That is, how much energy would it take to reverse the ability of matter and anti-matter to co-exist? Or would removing the curse release even MORE energy? Would removing the curse take up all the energy, so none would be left? Obviously, if matter and antimatter were kept from annihilating each other, it would take tremendous energy to do so.
As an example, would this curse be like bottling up antimatter and constraining it? Therefore, the antimatter in the sun would be equivalent to being in a very strong shell. The energy released by the removal of the curse could all go into destroying this shell. Think a contained explosion where all the energy is used up in destroying the container, vs an unconstrained explosion. The later would have far reaching consequences, the former would not.
Physics can not even answer the question as to what happened to all of the antimatter to begin with. Why didn't the universe just self-destruct at the very beginning? We just don't know enough about the process.
So physics could not answer this question. Therefore, it is entirely up to wizardry. There is no answer except a wizardry answer. This is so surrounded by wizardry, that whatever the author would like to happen, would happen, and whatever the author chose to happen, could be explained by a thick enough physics book.
I would suggest that, if the author wanted a really, really big bang, then the author would posit that all of the energy that went into the curse in the first place, in excess of the energy formed by matter-antimatter annihilation, that is all of the accumulated pent-up energy, was released at the time the curse was removed.
If the author did not want ANY bang, then the author could have all of the energy created by the sudden matter-antimatter annihilation go into reversing all of the cumulative effects of the curse, and cancel each other out. That is, it would be as if the curse had never been implemented in the first place.
Either could be 'supported' by physics.