Thanks for the update. Glad you got it figured out and fixed.
I had a similar high HR reading a couple times, except I was doing zone 2 and it suddenly shot up to over 200, while using hand grip HR sensors on a stationary bike. I subsequently had a panic attack, with dizziness, and my left arm and hand were cold for a few hours. That resulted in a number of tests, and the electrophysiologist & I both believed it was a panic attack, but during the testing, we also found a blockage (got a stent) and a Brugada syndrome diagnosis (neither determined to be related to the initial high HR episode).
As others have said, make sure your HR monitor strap sensors are moistened (water works fine for me). Sometimes, when I initially start my workout, I’ll get an abnormally high reading, and putting a little water at those 2 locations brings the reading down to where it should be. Also, if you have a pulse-oximeter, (clamps on the end of the forefinger) that you maybe bought during covid, you can use that to check any possibly errant readings, and it fits nicely in a pocket for road rides.