Feels like a general issue you your Discourse instance.
sam@arch discourse % bin/rails c
Loading development environment (Rails 8.0.4)
[1] pry(main)> I18n.exists?("js.user.username.short_instructions", :en)
=> true
Particularly something around build. During build we parse all of this stuff and pop it in tables, somehow this did not happen yet in your instance.
Would recommend a console rebuild.