When it happens, my first guess would be server’s caching.
Try removing it from OJS admin pages or with something like:
find /var/www/html/cache -type f -delete
If it’s not cache, I will check twice force_ssl=On and trust_x_forwarded_for = On as well as your mod_rewrite rules to ensure you include the proper redirections for the /api calls.
To help with the issue the logs from your missing hits trying to reach css and the frontend errors could also be helpful.