Since the stray, wither skeleton and husk got new sounds, they have no subtitles.
Polar Bear Sounds Not Showing up on subtitles
Husk/Stray sounds not on subtitles
The sound of the new mobs do not appear in the subtitles! MC 1.10
In 1.10, Strays, Husks, Wither Skeletons, and Polar Bears do not have subtitles.
Strays, Husks, Polar bears, and Wither skeletons "still" doesn't have subtitles