This may or may not related but is also a sort of bug. When you pack .dat files or .txt files (these are the caption files) in VPK, the game is not loading these files for some reason. It doesn’t matter if you upload it to the Workshop or not.
By the way, it does load the VPK itself, as when you pack it with textures etc it does load them all. However the game is simply ignoring all .dat files in VPK. Maybe this has to do with the bugs? I’m not sure why it works for someone and why it doesn’t though.