after more exploration, i see that files which are placed in the root of my site can be preloaded and seeked correctly when played via the video.js package or directly as a video element.
however, the videos that are played via the php application that runs my site are stored outside of the root directory of the site and i suspect that this is the cause of them not being streamed. (i am using the elgg social framework (www.elgg.org)). when i request one of these files from it's extermal/absolute url via curl, the accept_ranges header is not present, yet the header IS present when i place the same file at the public root of the site manually and request the file via curl.

e.g. the site is at : /var/www/html/mysite/
and the videos are at: /var/www/data/paths/

so the root directive for the server in the nginx config is /var/www/html/mysite/

what appears to be occurring is that the request for the mp4 file is being processed via php so is using the .php location in my nginx config instead of the .mp4 config... thus the mp4; directive is not being triggered for mp4 files.
when i place the mp4 directive inside the php location block i just get prompted to download a file whenever i navigate to a php page.

so this would appear to be a case of either needing some conditional logic in the nginx config file or a re-design of the architecture of the elgg php application. this was already highlighted in a related thread in the elgg community, though not to resolve this explicit problem - only to improve performance. (https://community.elgg.org/discussion/view/1653829/anyone-excited-to-team-up-to-create-an-enhanced-video-plugin-for-elgg)