Every once in awhile my watch will show something way off. Like it’ll read Z1 as I’m huffing and puffing up a hill or it’ll show Z4 while I’m just bebopping along a flat road easily breathing through my nose. For those instances, I go by RPE or glance at my pace and see how that pace aligns with my zones. Sure, my TP and my Strava data will be off for this session, but I’m willing to risk that in order to have a productive session.
On the other hand, if my HR is reading higher than normal and I feel crappy or sluggish, then it might actually be an accurate reading and I’ll dial back the pace/effort to stay in zone.