The video border, the title text outlines, and darker color of the background pinstripe are all defined as #ff4080. The subtitle’s outlines are also defined as #ff4080, but appear as #fb678a (plus or minus MP4 compression artefacts). What is causing this inconsistency?
My first guess would be a colourspace inconsistency.
See the section on that here: ASS File Format Guide · libass/libass Wiki · GitHub
We don’t handle actually rendering the subtitles, we just define an ASS file that the renderer or player will use, and really, it’s mostly allowed to do anything it pleases with what we define there (a bit like a brower can locally mangle your CSS to suit itself) but most things these days use libass and behave like it does.
So if fixing the colourspace doesn’t fix this, you’ll probably need to look at what is rendering it next.