Fallback_locale and .json translation files, doesn't work like .php files

[djped]

Laravel version 8.6.4
Php 8.0.10

Have an app that I've just switched from using
resources/lang/en/messages.php type files to using a single
resources/lang/en.json file.

When I use .json files for translations the fallback_locale set in app.php is not being respected, I'm getting the key of the strings instead of the fallback locale translation string.

Using pseudocode to describe the issue....

lang/en/messages.php
	'hello' => 'hi there!'
setLocale('ru')
__('messages.hello') >>> returns 'hi there!'

This is what you'd expect, the fallback translation because no lang/ru/messages.php exists.

lang/en.json
	"messages.hello" : "hi there!"
setLocale('ru')
__('messages.hello') >>> returns 'messages.hello'  

You would expect 'hi there!' as this is the fallback translation language.

ANyone have any ideas? Is this a bug?