Thanks for testing it! I have done some further testing, turns out it's the order in which default plugins get loaded.
Can you try to test by setting your extension_enabled settings in your db like so:
["flarum-emoji","flarum-flags","flarum-markdown","flarum-mentions","flarum-tags","flarum-subscriptions","flarum-sticky","flarum-suspend","flarum-likes","flarum-pusher","flarum-lock"]
This should allow you to reproduce with the given steps.
Loading flarum-lock at the beginning works, I cannot reproduce it when configured like this:
["flarum-lock","flarum-emoji","flarum-flags","flarum-markdown","flarum-mentions","flarum-tags","flarum-subscriptions","flarum-sticky","flarum-suspend","flarum-likes","flarum-pusher"]
No third party plugins required!