I have developed it from scratch few years back. However, I don't think the theme would be a problem - as I wrote above, I read online that this is a common issue related to the Metatag module and that one has to call the methods above - but that's PHP, so it would be usable in D7 .tpl files, not in TWIG in D8 - and that's the question, I think: how to call this in a D8 TWIG template.

Ping :) I'll be grateful for any advice on this.

Thanks, zhilevan. I edited the question and added the code.

Silly mistake. I did that, but still nothing.

yes, I renamed the function to mytheme_hook_node_view, where mytheme is the right name of my theme, but still no difference, the metatag does not appear in the code (of course, clearing caches)