If you are attempting to do a custom workout using pace, then you need to be on a flattish route where you can better manage pace. On hilly runs I'll use HR.
The problem is that the workouts use average pace for the lap, which means if the intervals are short, and you were previously running at a considerably different pace, it takes a reasonable amount of time for the average pace to catch up to your current pace, especially if you are using a treadmill. As far as I am concerned this makes the pacing, especially for intervals, as good as broken. Workout pacing needs to be able to be switched to actual pace instead of average pace, I do not know if that is possible.
I emailed TrainingPeaks support and here is their reply:
Thank you for reaching out. Unfortunately, there is no way to disable the pace alerts at this time. This is a limitation with Garmin devices and we do not have any control over this functionality.
However, I can tell you that pace will not show on the device the workout is synced to when you use RPE. I often use this on my workouts because I hate that I cannot disable alerts on my Garmin when I am outside of the pace zones, so this is a nice workaround since I know my goal paces.
I have not yet tried it but it makes sense. I just have to figure out how the scale of 1-10 RPE maps to my current pace range.