Issue with MPC-BE + madVR + LAV + XySubFilter

[beto@endrix]

So yesterday I found out just how much better this combo is compared to VLC, and spent my night configuring it (following this and this) and getting it ready to watch a BluRay 4K movie today.

However, I stumbled on a problem: the movie is in a matroska container and it has forced french .pgs subtitles coded in, although it has many others in the same file (forced and not). When I switch to another language, or when I set my preferred language to play at start, as soon as the first subtitle appears, the video loses all its quality, the image appears really dull and washed. I think that happens because the renderer swaps to internal, or madVR crashes. Maybe it's none of that, still new to this.

Can someone tell me what is happening and how can I fix it?

Windows 7
1070 on the latest drivers

Happy to provide more information if you need.