It’s all my fault! It was me who requested change from something like “dot matches newline” to “regexps match newline” because I got confused several time using regexps like [^a-z]. And it was different depending on whether I searched in single or multiple files.
Now [^a-z] and s match newlines if “regexps match newline” is off, but only if “additional lines to search …” is not 0. If you set “additional lines to search …” to 0 you should get single line searches. I am not sure though, this is confusing.
Perhaps this is a bug and “additional lines to search …” should be disabled when “regexps match newline” is disabled?