iBooks look-up works differently after 10.11.6

After update my MacBook to 10.11.6, the function of "look-up" in iBooks changed.

Before, if I double tap on one word, it will automatically look-up it in dictionary and wiki.

After update, when double tap, it only show wiki, but not in dictionary. (For some words, it will leave an "open in dictionary" button. This causes quite some inconvenience for me.

Is this bug of the software, or could it be configured through options?

If it's a bug, how to report it to Apple, so to have it fixed?

Thanks