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Combustion of of $0.255 \space g$ of isopropyl alcohol produces $0.561 \space g$ of $\ce{CO_2}$ and $0.306 \space g$ of $\ce{H_2O}$. Determine the emperical formula of isopropyl alcohol.

I'm not asking how to solve this problem, but why this doesn't violate the conservation of mass? Shouldn't the initial combusted mass equal the sum of the resultant masses?

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Because the problem statement indirectly includes an additional reactant.

At an introductory chemistry level, the term combustion implies reaction with $\ce{O2}$. Thus, your missing mass should show up when you introduce a stoichiometric amount of oxygen to your reactants.

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