Why did the response code of a non-existent page stop being 404?

Question

Search Engine Optimization

Hello!
According to one site analyzer, the following situation arises:

It jumps constantly, then the server's correct response, then it's wrong.
CMS joomla.
The question is: what can it be connected with and is it scary?
In fact, the response code of the server of a non-existent page is what it should be - 404, I don’t know where it takes the opposite from.
Thank you.

Answer 1

It is necessary to look in the logs which requests are checked and what is the real answer.
Perhaps under some circumstances you really do not have a 404 response, but 403, 500 or 200.