Yes, it's only working for the internal filter at the moment. The problem is known and will probably be fixed at some point. (It may work in MPC-BE if you want to test that.) The internal subtitle filter is usually regarded as sub-par, though. You may notice slightly different colors and other problems compared to xy-vsfilter or XySubFilter. It can also be slower (that said, the devs have done some work on speeding it up recently).