Subtitles being "burned into" the frames instead of being a separate track, also known as hard-coded sometimes. This enables one to use subtitles on devices which cannot traditionally use them or screw up the display. But this means the server needs to re-encode each and every frame, which is a massive load on the server.
That setting also takes host names. As long as both containers share at least one network, put in the service name (not the container_name!), e.g. "npm" or whatever yours is called and you should be fine.
Those constructions rely on all parts being where they are, otherwise the whole thing collapses. You'd need a different kind of bridge for the single stretches to be independent.
So not skewed at all...