Sometimes (quite often actually), the animation on the tab bar is not correct. Most the the times, the blue underline appear, quickly disappear and re-appear with the animation.

They're working on it (I hope) :)
https://github.com/google/material-design-lite/issues/954
Damn I searched before but could not find any issue of it...
Thanks!
Anyway, the other ticket suggests to refactor the anim to match the "siblings anim" in the spec (which I agree).
Closing as duplicate of #954.
I write more detail to express OP's mind
This is the slow down GIF:

When I mouse up, The underline start to animate immediately, no showed suddenly
There is a 0.01s delay in CSS, write by #311
Oh wow, the gif slowdown makes it much clearer as to what is happening. I thought this was just an issue about it not being by-the-spec example animation. Thanks for that @zcyzcy88.