Worked out that this is not an issue with Obsidian but with a selective company firewall. I’m leaving the thread up in case someone else comes across a similar issue. The firewall allowed Obsidian to collect a plugin’s manifest.json but not any other plugin files. The resulting plugin therefore existed, but in an incomplete state.
To check whether this causes your “failed to load” plugin issue, open your vault’s .obsidian/plugins/[plugin_name] folder. A complete plugin install should include, at a minimum, the files main.js and manifest.json. If any plugin files are missing and you want further confirmation this is a firewall problem, go to the plugin’s page on GitHub and open the latest Release page (link in the right-hand sidebar). Attempt to open or download the missing file(s). If you are blocked by a firewall, there’s your problem. (Deep, disappointed sigh.)