I have found wrong heart rates values (lower than expected ones) during (outside) running activities since 19.XX firmware version. But I have found also that my HR values were OK during treadmill activities.
So, I thought it could be something related to GPS settings, and went to the settings section on my watch, and changed the default "Auto Select" setting to "GPS Only". As a result, my HR values are better now.
I do not know why, but at least y my case, the Auto Select mode (SatIQ technology) was interfering with the OHR.