By default (without changeing browser identification/user agent) Opera browser on Linux is always detected as mobile browser. And so redirects to MantisTouch. It also redirects to MantisTouch from Wiki link.
It's annoying behavior for mantisbt.org site

Reasons are following:
For header
HTTP_USER_AGENT = Opera/9.80 (X11; Linux i686; U; ru) Presto/2.10.229 Version/11.60

this parts of code
$t_mobile_ua = substr( $t_agent, 0, 4 );
...
$t_mobile_agents = array(
...
'oper'
...
detect Opera on Linux as mobile browser
but for Windows version flag is reset later
...
if ( strpos( $t_agent, 'windows' ) !== false ) {
$t_mobile_browser = false;

Workaround is to change "Browser identification" to "Firefox" or "Internet Explorer"

also commented on github:
"up.browser|up.link" is not strictly correct for preg_match
"up.browser|up.link" is better

here is the patch to fix this

diff --git a/core/mobile_api.php b/core/mobile_api.php
index 1dd77c4..d5a180b 100644
--- a/core/mobile_api.php
+++ b/core/mobile_api.php
@@ -35,7 +35,7 @@ function mobile_is_mobile_browser() {

$t_agent = strtolower( $_SERVER['HTTP_USER_AGENT'] );

• if ( preg_match( '/(up.browser|up.link|mmp|symbian|smartphone|midp|wap|phone|iphone|ipad|ipod|android|xoom)/i', $t_agent ) ) {
• if ( preg_match( '/(up.browser|up.link|mmp|symbian|smartphone|midp|wap|phone|iphone|ipad|ipod|android|xoom)/i', $t_agent ) ) {
  $t_mobile_browser = true;
  }

Marking as 'acknowledged' not resolved/closed to track that change gets ported to master-2.0.x branch