@dydaevskiy You didn’t get much response (none) the first time you asked this question and haven’t yet received a response this time… So here is mine…
TLDR: It has to do with the quality of your provided <title>, not the Grav software.
This is a very old recurring SEO question. The issue of “changed titles” in the Google search results pages has nothing to do with Grav or any other CMS. Google’s algorithms change the titles at will. This mostly happens when the title provided by the webpage does not, according the algorithm, match the content of the page in combination with the query posed by the user. They attempt to provide a title that is as meaningful to the users as possible.
In short: It has to do with the quality of your provided <title>.
For example, the page you are referring to contains 38 times the word ‘Grav’. It is also mentioned in the first <h2> on the page. The page also contains 118 times the word ‘CMS’.
An other page: https://techblog.sdstudio.top/cms.
Google provides it the title ‘Grav CMS’. Looking at the code, it contains a very simpel one word title: <title>CMS</title>. How descriptive can a single word be? It also contains 38 times the word ‘Grav’ and plenty of ‘CMS’. This is a strong candidate for Google to alter the title in the search engine result page…
The following video from Matt Cuts from Google (2014) explains the issue. In the comments are references to more docs.