Well done on the result!
An erroneous heart rate reading (usually high) at the start of ride is not uncommon.
The usual cause is poor electrical contact between skin and chest strap, and the strap can pick up stray electrical signals from static electricity in the technical fabrics of your jersey. This is more common as the weather starts turning cooler, and your skin can be dry at the start of a ride.
Pre-moistening the sensor pads on your chest strap is normally the recommended solution. Once you are 5-10 minutes into a ride, your natural sweat starts taking over, as you can see from your graph where your HR settles in at a steady level.
Speaking of sweat, over time salts from sweat can accumulate in the fabric of the strap, making it easier for these stray electrical currents to travel through the strap. Regular washing can help reduce this salt accumulation. Or even just a quick rinse in fresh water at the end of each ride.