Why is magnesium sulfate acidic? Why are my calculations showing that it is basic?
Your answer lies not in the solubility product but in the acidity constants of sulfuric acid and the basicity constants of $\ce{Mg(OH)2}$, though the solubility product may well come into play. You would need to solve a system of 6 equations, one for each of the:

2 $\mathrm{p}K_\mathrm{a}$ values for the acid
Two $\mathrm{p}K_\mathrm{b}$ values for the base
Proton condition or charge neutrality
And as you will be groping in $\mathrm{pH}$, one to be sure you don't exceed the solubility product.

As sulfuric acid is 'stronger' as an acid than $\ce{Mg(OH)2}$ is as a base, you would expect the answer to be slightly on the &quot;acid side&quot; of neutral $\mathrm{pH}$.