After I received the 18.23 update, I went for my morning run the following day using my Garmin H10 heart strap. During my base run my watch was reporting my heart rate dropping below the lower threshold, even though I was running at my usual place. This occurred three times, during the run.
I then went for a gentle walk with my wife. My watch was using the optical sensor and it reported a heart rate off 152bpm which is near my lactate threshold during the walk. There is something definitely wrong with this update!