Been experiencing the same thing, I noticed especially after timing my attack rotations with a stopwatch. However, I also rarely experience the full 20% bonus. It is very inconsistent, and I'm not sure what causes it.
Other folks mentioned certain weapon actions and affixes (S4: Escalating Pursuit) will activate the chain. For weapon actions, I know Double Saber's Whirlwind will activate the chain.
Another thing I've noticed is the Chain Boost buff refreshes the strongest bonus. If you end the chain with a low combo while having a 10% bonus, it will reset the timer with a 10% bonus. Even if you have a chain of 1 hit, it will refresh the 20% attack rate increase if you manage to get it. Not sure if this one's a bug, but it's something worth knowing.
I believe they lurk the forums here too. But if you really want them to know something, check out the 'CONTACT US' and you can report things directly.