Update: correction - it appears the selected search engine is in fact 'sticky' between sessions. However if your preferred search engine is not listed the speed dial solution is the easiest way.

My understanding is Google will be the default at the start of each session. If you use a different search engine by selecting from the drop down list, it appears to continue with that until you exit opera.
The easiest option seems to be to put your chosen search engine on the speed dial and go to the search provider first before entering your search text.

What if you just type the address without the ftp:// part?

Thanks for that suggestion because it worked. After a bit of testing, each time clearing browsing history, it appears:
ftp://+address ->google search for ftp://+address
ftp address only -> ftp site
usual web address with or without http:// -> website

Also suggested sites before pressing enter appear to come from browsing history, which is much better than I previously thought. I still suspect browsing history is going to google 'on the fly', not an issue if it doesn't include credentials like names and passwords.
So thanks for your suggestion, the bottom line I guess is hopefully the opera devs could check/ensure that such details can't be passed to any search engine.

What FTP site? I visit Opera's FTP site ftp://ftp.opera.com/pub/ regularly and haven't seen this issue ...

It works on Opera for PC but not for Android, at least not on my Opera for Android. Wifi or mobile data connection makes no difference. I'm not selecting suggested results, and at the end of the address I'm entering the 'Enter' key on my android keyboard rather than pressing the google search button.

It does a google search of the ftp address rather than go to the ftp site, unlike my pc (including your quoted opera's ftp site). At best I can go to the ftp site from the google result. As far as I'm concerned this is unsatisfactory - given the possibility of inadvertently passing your login details to google.

In fact everything is going through google search - including http addresses - because as you type an address google is suggesting possible results. If you don't select a suggestion, and have typed an address correctly, there's a slight pause as google matches your entry to a single website, then passes you to it. In other words, you're always searching, you just don't see a load of results if google finds a match.

I hadn't noticed this before. I think I'll test older versions of opera for android and failing that, sadly I'll have to look at other browsers (though I'm not holding my breath on those either).

Typing in ftp://..... searches google for that phrase instead of going to the ftp site. Not only that, if you had entered your name and password in the address you would have passed that on to google and potentially the whole world. Clearly, the combined search and address bar is potentially very dangerous.