Problem with determining the language of a post

Polylang is set on my site to find out the page language by the desired content (single, page, category, etc.) It works well for except single posts.

I found the problem is that get_query_var('lang') always returns my default language slug (hu) on single post pages. It works well on categories, tags and the home page tho.

There are four conditions in get_current_language() (core.php) that determine the language:

[core.php:206]
if ($var = get_query_var('lang')) // 1st
...
elseif ((is_single() || is_page() || (is_att... // 3rd

If I place the 3rd condition before the 1st then it works, since it finds the language from the language of the post, not the query_var, but I still don’t really understand why is the query_var always “hu” for every post, the real problem lies somewhere there.

I know a few weeks ago it was working properly, then I updated and this was the case.

Please help!

https://www.ads-software.com/extend/plugins/polylang/