Is there any law for pH conservation in chemical reactions?
No, there is no law requiring pH conservation. And such a law would be chemical nonsense. Why? Read on.

You need to recall what pH actually is. It is defined as the negative base-10 logarithm of the activity (similar enough to concentration for almost all practical purposes) of positively charged hydrogen ions in solution.† Or in equations:

$$\mathrm{pH} = -\lg [\ce{H+}]$$

Note that most conservation laws in physics somehow relate back to the conservation of energy or something related. However, the pH of a solution cannot be easily traced back to any state function or anything else that would warrant its conservation.

In fact, the only thing conserved is the atom itself. Hydrogen, if $\ce{H+}$ is somehow present in a reaction or solution, does not disappear, it needs to be put in on one side of the equation and turn up somewhere else. In reactions that liberate protons of any kind, usually bases are added to capture them. In those that require protons, acids are added. What (if anything) is conserved is the number of hydrogen atoms but if they are not active as $\ce{H+}$ in solution they do not count towards pH. Therefore, there is no sense in postulating pH conservation.



†: $\ce{H+}$ as is does not actually exist in solution. One can treat it as $\ce{H3O+}$ for all intents and purposes, but actually it is better to think of it as a proton being shuffled back and forth by multiple water molecules. I think I recall four being stated as the lowest count in undergrad education.