I have changed how HRV data is stored in Intervals.icu. It now associates each tuple with a timestamp so it maps back to the power and other data (tx @John_Peters_endura.f for the tip). Unfortunately you need to reprocess old files to get this.
This is what the CSV download looks like now. The last column is the HRV data with a ‘:’ between each beat to beat ms value:
On another note I noticed that my Garmin Edge 130+ stopped recording HRV data recently. I suspect letting it update undoes it. Have to repeat this process: HRV-Guided Training - #19 by david