The HR target zones displayed in the workout player ALWAYS match the corresponding target Power zone. So, if you should be riding with Z4 Power, the workout player will always show the corresponding Z4 HR zones. It doesn’t matter if this is the first second of an intense 20 second AC effort, the 90th second of a moderately intense MAP effort, or 5 seconds of an NM sprint effort.
This doesn’t mean that your HR should always match. Far from it. The key to the workouts - as mentioned before in this thread - is to match the target Power (and sometimes Cadence) except for the constrained effort in Half Monty. And then your HR will follow.
As has also been pointed out in this thread, your HR will always lag behind your effort - especially for short intense intervals. You can’t expect your HR to catch up that quickly. And then when dropping back down from a long intense effort your HR will lag on the way down, as well.
And also, if you reduce the intensity of your workouts, the HR zones do NOT adjust. So, if you lowered your workout to 85% intensity so that all your Z5 efforts became Z4 efforts, the workout player will still show the HR target zone that corresponds to the 100% intensity power target.
So, as previously mentioned, as long as you have a smart trainer and/or a power meter, unless you’re doing the Half Monty constrained effort, just ride to the displayed Power targets and ignore the displayed HR target zones. They are there for reference, and for review after your workout, but there’s little use in trying to match them due to all the reasons previously mentioned.