Update after field test (May 2nd): I have now confirmed that the real-time ZoneSense gauge on the watch itself (Vertical 2 / firmware 2.53.42) frequently shows Yellow (Anaerobic) during an easy run at avg 122 bpm — well below my Aerobic threshold (S1 = 130 bpm). This rules out a pure app-side recalculation bug. The issue appears to originate in the firmware 2.53.42 DDFA computation, which was the first version to include ZoneSense as a native function on Vertical 2. The app 3.9.1 seems to faithfully reflect these erroneous watch-side values, whereas the previous app version was apparently applying post-sync correction/smoothing that masked the problem.