It appears that, after equipping the FR0ST-EE5 and activating Rapid Cooldown the first time, your discipline stat becomes broken. If you aren't sprinting with the exotic boots on, no matter what your discipline is, even if you change armor, your grenade cooldown will always be 60 seconds, as if you had 0 tiers of discipline. Sprinting with the boots on does indeed reduce this cooldown to 30 seconds if you sprint continuously, but that's a slower cooldown than tier 5 discipline without them. As far as i can tell, the only way to fix the problem is to return to orbit.