When I do an advanced search on Minibb 3.3 with:
Search for = "lsjhdflsdf"
Fields = "Topic titles"
Username = "Rider"

I get 10 hits (well, ok, 11 hits including this post) where I would have expected to get 0 hits because I never typed the word "lsjhdflsdf" in a topic title. So it seems to me that there is an AND missing.

If I remember it properly, when you search by a username, whatever typed in the search phrase is ignored - that way only posts or topics created by a certain user could be found. But I'll take a look during the next working week.

That is indeed what happens: the search phrase is ignored. Next question would be: is this how you want(ed) it to work?

Yes, it could be like it has been planned intentionally; but I will check. There's no a big deal to add AND condition for the next version, even if it's not present now.