Couple things, what were you doing then, and are you relying on your HR monitor to be accurate all the time?? So many monitors are wonky, work most the time and then have glitches all over the place, especially wrist ones. I really dont see anything too crazy, looks like at the beginning of your first hard effort that your HR is deciding where to level out at. Or wonky readings when you just start to go hard too..

almost looks like you were free spinning for a good period of time before the area you are referencing. Maybe it's just the HR monitor trying to play catch up and figure out what's going on once your HR went from practically a resting state (during the free spinning) when you weren't putting any power out and then going hard quickly. Many HR monitors I have noticed will drop into a sleep mode and such when you get too low and then freak out until they stabilize. I see this same thing if I'm dong like hill repeats or the such (running) and my HR drops low and then I go hard but the HR monitor gives crazy readings and then locks in.

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Cheers, Ray

The question is, do you feel abnormal at the time of that change in the graph? or are you only seeing this afterwards when you analyze the ride?

Would suggest to try to reproduce the HR blips while working out with two HR straps connected to two different sources and compare the graph outputs.