I even see negative decoupling on 40-50 minute intervals at upper sweet spot / threshold intensity. For example 3 years ago around this time my aerobic fitness was very high. I was doing longer ss/ftp intervals on an outdoor segment, take a look:
And yes my FTP was set correctly, those IF values are good. For reference my LTHR is 162bpm. Without looking I’m sure some of the lower HR were due to functional overtraining as I was going really hard a lot of the time.
On that other thread I speculated (based on Alan Couzens article) that despite doing some over threshold work on warmup, some less efficient fast twitch fibers were still being used early on in the interval, and then most of the fast twitch stopped and more efficient slow twitch fibers took over. In other words, my slow twitch went “all in” and that was reason for negative decoupling. Or its because I was training so hard around that timeframe, and my HR response was blunted. But honestly I have no clue as to why, it just is what it is.