Okay that’s really strange … In fact, the loop-forums.php is not ignored at all, it’s just that it changes nothing.
Because if I delete the original loop-forums.php in \wp-content\plugins\bbpress\templates\default\bbpress, the forum is not displayed anymore then if I move the loop-forums.php from extra to \wp-content\themes\xxx\bbpress it is displayed again.
So yeah, the loop-forums.php isn’t ignored, it changes nothing.
But I checked, they are both different so I don’t understand why they both display the same result … And of course I closed my browser (and I tried to display with Internet Explorer too, cleared cache, …).
But I’m runing my blog on WAMP Server, I’ve no idea if it the issue origin ?