This site gives a #K_"sp"=5.16xx10^-11#. However, in acidic solution, we substantially reduce the concentration of one of the reagents that participates in the solubility expression, i.e. we modify #[F^-]#.
#F^-# is moderately basic (i.e. solutions of potassium fluoride gives rise to basic solutions), and would tend to be protonated in acidic solution. If we were willing to solve 2 equations in 2 unknowns, we could work out the solubility of #MgF_2# to a good approximation.
And thus #MgF_2# is expected to exhibit greater solubility in acidic solution.