Hello,
When Xenforo calls ImageMagick (PECL version, fwiw), it calls the 'thumbnailImage' function instead of resizeImage (or adaptiveResizeImage).
(resizeImage is only used by Xenforo for Scaling Up)
This can be seen in the php file
\upload\library\XenForo\Image\ImageMagick\Pecl.php:
foreach...