Its open-source, nothing prevents you from stepping in and fixing it yourself.
I had no problems with subtitles, and can't find any issue related to it that isn't years old. I'd say if there's an issue its either unknown or low priority from the low demand to fix it.
VLC probably has a lower barrier to entry than some projects, but this advice still grates every time I hear it. There's a lot of things that could potentially stop someone contributing to VLC, even the ability to build it from source.
I had no problems with subtitles, and can't find any issue related to it that isn't years old. I'd say if there's an issue its either unknown or low priority from the low demand to fix it.