I can confirm this behaviour (Version:65.0.3467.48).

It is not only a Linux issue, the same happens on a Windows machine.

I inspected the traffic of Opera browser with Wireshark, and what could be seen is that after each new character that has been typed a new request to the server was sent (which was not shown in the Debugging tools).

In my opinion this is not only a small configuration problem. It is a SERIOUS ISSUE of Opera browser, causing a CONSIDERABLE RISK of unexpected behaviour on client and server side.

Just imagine, a developer wants to test the following service and manually types the URL:
www.server.com/delete_user.php?id=2468

While typing, the browser will send each (incomplete) URL to the server, already causing actions there:

[httpd] /d
[httpd] /de
[httpd] /del
[...]
[httpd] /delete_user.php?id=
[httpd] /delete_user.php?id=2
[httpd] /delete_user.php?id=24
[httpd] /delete_user.php?id=246
[httpd] /delete_user.php?id=2468

So, what happens: the server will first respond with many 404 errors, but then deletes successively users 2, 24, 246 and 2468. When the developer then hits Enter in the Browser, again a request (now with the full URL) is sent. But at this time user 2468 is already missing, which probably gives again an error ("User 2468 is not in the system").

This bug is hence confusing and very dangerous, and needs to be fixed as soon as possible!