I have used a couple of optical heart rate monitors and the ones that go around my skinny wrists seem to struggle, They often go to a progression of what the correct number will be IE jump from 120 to 160 or 150 to 180, also when running they can get confused by the cadence I am swinging my arms at.
“Not asking you to do doctoring over the internet, but is this a bad thing? Should I avoid going hard? Is there danger to an abnormal heart rhythm?”
Tachycardia or racing heart is a thing but as you had a heart assessment a year ago I would say highly un likely.