That activity has a lot of broken HR data towards the end. If you Actions → Settings you can tick the ignore HR box and it will be left out of the compare data set (I just did that now).
Intervals.icu computes power vs HR data by breaking the activity into 1 minute buckets with average power, HR (adjusted for lag) and cadence for each. Buckets with stopped time and other dodgyness are discarded. This is stored in 5w increments. So for 200-204w your avg HR (for all 1 min buckets with this power) might by 142 and there were 32 minutes (buckets) in that range.
The compare page calculates a weighted average of each power bucket for all the activities.
Your power vs HR lines are not nearly as straight and linear as mine. Not sure why that should be?