Why is the plugin not localized?

Question

WordPress

Hello. Maybe someone faced a similar problem: I
installed the Loco Translate plugin and let's translate the theme with the plugin. The theme was translated perfectly, without any problems. The plugin does not want to be translated at all. The plugin adds a custom post type and metabox. I want Russian inut headers to be in this metabox, but it doesn't work.
But the strangest thing is that one inscription in the plugin was still translated. Having figured it out, I realized that there is absolutely the same in the topic itself. What, now add all the inscriptions from the plugin to the theme?
Thanks in advance.
PS Both the plugin and the theme are packed with __() and _e() functions.