Did some quick testing on the 5 star GC leves at Halatali. Every mob was level 52.

With Rampage up, Steel Cyclone crit damage ranged from ~500-1000 against the same mob. What caused the difference?

When only one mob was attacking me, Steel Cyclone did 500-600 damage. When I had 3+ mobs attacking me, Steel Cyclone did 800-1000 damage. Now take a look at the patch notes on Rampage:

<Before>
Increases your attack speed and power for each physical attack against you, and restores HP for each critical hit you land.
<After>
Increases your attack speed and parry rating for each physical attack against you, and restores HP for each critical hit you land.
・Effect now wears off at a faster rate.
My guess is that the effect on Rampage decays so quickly that it can never be fully stacked when fighting only one mob. Thus, you will never get the full bonus damage to Steel Cyclone unless you are tanking multiple mobs.

This is all just eyeballing of course. Wish I had some numbers pre-patch so that I could compare. If someone wants to do better testing, please feel free to do so.