Just a quick check if I misunderstood something, but by my understanding, the PR chart should show a decline as my best power I held for 2 mins is definitely also AT LEAST my best power for 1 min 30 seconds etc. I didn’t really look for it, but just happen to come across 2 examples:
If every point on the curve was taken from a max effort then you would expect it to fall as the duration increases.
However, not all of your PRs are a true best effort for that time.
For example, imagine you did
1min at 300w
1min at 100w
1min at 300w
Your best 1min power = 300w
Your best 2min power = 200w
Your best 3min power = 233w
Over time if you do enough hard efforts of different durations the PR curve should approach the shape you would expect it to take.