I do not believe that there have been any changes to the arrow rate of fire. I have always had trouble getting my bow to juggle. It may be a matter of how high you initially launch the enemy, how quickly you fire your arrows or the timing with which you fire your arrows. I tend to think it relates to the last of those as that would be the hardest to keep consistent.
A few points to consider that may explain why things may seem to have changed. Perhaps your execution of the combo has changed, but it could also be situational. If you are not recreating the situation that generated the combo before, that would explain the discrepancy.Things to consider are: if you are using the same abilities that you did then; if you are facing the same enemies; if your character or the enemies are of a different level than when the combo worked; if the enemies have a different amount of health than when the combo worked; if there are any effects on yourself or the enemy such as blessings or poison or bleeding or enflamed or frozen that differ from when you performed that combo; if you are using better or worse weapons than when you performed the combo; if you upgraded any particular abilities or skills that may affect damage done or some other aspect of this combo's execution; if the terrain where you performed the combo was accommodating to the type of juggling you are trying to perform and you are no longer on such terrain; if you are not in a similar or the same relative position to the enemy as when you juggled them; even a twist of fate that provides certain boosts may have an effect if it somehow alters damage or criticals or status effects that may or may not have some bearing on the animations or enemy reactions to taking the hits from your bow.
I haven't seen any updates or noticed any such changes and I use the bow quite often. Unfortunately 38 studios doesn't seem to like the idea of release notes so I cannot be authoritative beyond my own personal experience. The only suggestion I have for an authoritative answer on whether something was actually changed in game would be to ask in the Kingdoms of Amalur Community Forums in either the General Gameplay Discussions or General Discussions and hope that the devs give you an answer although I expect you will more than likely receive many more community answers similar to this one before the devs will tell you anything if they reply at all.