When viewing HDR videos with SSA/ASS subtitles, it looks like the SDR subtitles aren’t blended with the HDR video correctly. The following three images show an example of the exact same subtitles on SDR and HDR versions of the same video.
I don’t know what library you use to render subtitles, but libass has some possibly related discussion: add ASS header field "transfer function" (to define HDR vs SDR etc) · Issue #297 · libass/libass · GitHub