Like Scott Semnple I may not have read your post carefully enough.
You are using pace but you are doing these on some uphill course right. So you are not really using something like a target pace from a race if I understand correctly. Normally when we refer to using Pace as the measure of interval intensity we do this by taking some recent race or time trial effort to establish the current pace you can race at. Then we’d use that race pace to set the interval pace. For building endurance you’d do longer race pace or slightly slower pace (depending on race length). For improving speed we’d use paces faster than race pace for shorter reps (again the lengths and pace depend on target race distance).
In mountain sports pace goes out the window so we are stuck using HR as proxy for effort.
I suspect what you mean when you say that you are using pace is that you are using a set distance on this hill and that to maintain the time you set on reps 1-2-3 for reps 4-5 your HR goes higher than it did on those first 3 reps. Is this correct?
If it is then you are correct the issue is probably more central Cardiac output related that it is local muscular fatigue.
I’m not a fan of the Friel zones systems as I understand them. They use arbitrary % of usually Lactate Threshold. That arbitrary parsing of your metabolic response so finely (some of these 7 zones are only a few beats wide) is what is called in engineering “False Precision”. There is no way this precision can be assumed even if one has a careful lab test.
This is why we use a simpler zone system with the principle anchor points the AeT and the AnT. Do you really think there is meaningful distinction in the metabolic or global training effect between say 176 and 179 bpm HR. Unlikely.
I would not get overly concerned with the HR increase you are seeing int he later reps.
The way many top athletes in XC skiing (where pace is irrelevant) use interval training is to go as hard as they can for the prescribed time and number of reps. It turns out that long reps like 10-20 min allow only about Z3 intensity. Medium lengths like 8min will allow Z4 for the well trained, Short reps like 4 min allow Z5 effort. All these are done with a 2 minute rest interval typically.