here

This paper is about Europe and thus does not shed much light on al-Majriti. The paper is about a metaphysical principle, not an empirical one. People point to al-Majriti because he might have done relevant experiments.

After thousands of years of acceptance of this metaphysical principle, Occam was skeptical; perhaps the original application of his razor was that "quantity of matter" was meaningless, and thus too its conservation (though atomists could use a count). Later 14th century Europeans defined "quantity" as "mass," and thus made a precise claim. Following Averroes, they used inertial mass, not gravitational mass.

So you could say that Lavoisier did not discover conservation of mass, but experimentally checked it. But he made a big splash at the time, probably because scientists were no longer so enthusiastic about metaphysical speculation. In particular, he showed the 14th century people made the right guess.