I had a look and I think your HRM just went a bit stupid tbh. It goes really high early on and then there’s a bit where it doesn’t fluctuate from 186 for almost half a kilometre.

Just the way it’s tracking looks a bit weird to start with as well. I’m sure I read somewhere that you can actually get some sort of gel or something to put on the pads on the strap to help get a better reading(?). Almost looks like that might be what’s happened here with the janky readings early on. I just lick mine, rank, but I’d rather spend my money on Chamois cream.
Not to say that doing some intervals to find your max HR would be a bad idea, like.