I think it is a bug in the new version that causes the button to be disabled if it is the only one there.
I have found that for videos that have another button in that corner such as those with subtitles (captions?) or the airplay button, as thedukeinpa notes above, the speed button works correctly.
The speed you set when the button is working is remembered on videos where it is disabled so switching to a video with captions, setting the speed then switching back could be a kind of (inconvenient) workaround.
I have a support call logged for this so hopefully they will come up with a fix rather than just declaring it to be the new intended functionality.