COROS Pace 4 HR still dropping during HIIT (2nd test) - any real fix besides HRM?

Second workout with my COROS Pace 4 (HIIT indoors, Ski-erg, very sweaty). HR starts normal, then either flatlines (~120–130) or drops suddenly during high effort intervals, even when I know my HR is 160–180+

Tried:

- Wearing higher on wrist

- Tightening strap

- Trying inner forearm position (actually worked briefly, then dropped again)

Adjusting mid-workout didn’t fix it.

Is this just a limitation of COROS optical HR in sweat/HIIT, or is there something else I can fix (besides buying an external HR monitor)?