Not sure if it was a recent update or what but I noticed after my run today I logged plenty of time in z3 and z5 but almost no time in z4. I looked after and my z4 range is 154-155 bpm. Contrast that with z3 (141-153) and z5 (>156). I checked the user profile on my watch and it says z4 is 93-95% based on %LTHR.
Any suggestions?