I wanted to inform people here aswell, while at the same time hoping to get Blizzards attention.
It seems the 5th tick doesn't work when you are above a certain haste point, which looks to be around 14.3% haste.
Here is the info regarding my tests:
All tests are on a Raider's Training Dummy.
I am Using Screaming Tiger, Qiang's Unbreakable Polearm
I used no trinkets other than Windswept Pages which I needed to get near the supposed FoF haste cap. To get to the different haste percentages I unequiped some gear, because of this I missed or got dodged sometimes, but ofcourse I counted that as a tick aswell.
I had my two own buffs up (5%crit and 5% agi,str,int), no other buffs or debuffs.
Before every FoF I jabbed a couple of times to get the trinket proc, then made sure I had no mastery procs up. Then I waited without melee'ing untill I was out of combat, and then did a FoF (while trinket proc was still up ofc).
The haste values (% and rating) are what I saw on my character tab. Not the total haste (cause that includes the 40% from using a 2-hander) but the line that says for example: "Haste 2,744 (+6.46% Haste)"
The 2 haste ratings that are on the edge of 4 and 5 ticks I tested multiple times, with the same result every time.
(for some I forgot to check the rating)
14.16% (6018) -> 5
14.19% -> 5
14.23% -> 5
14.26% (6060) -> 5
14.33% (6091) -> 4
14.42% (6130) -> 4
14.46% (6145) -> 4
14.50% -> 4
These results were consistent every time. So I do believe this problem has to do with haste.
I hope the devs can look into this, especialy because this is supposed to be our biggest damage ability and our T14 2-set bonus is designed around this ability.
ps: I also submitted a bug report ingame.