Insanely High HR

The "standard convention" is just the average of 28 year olds - like most averages, it's actually made up of a broad spread of individuals. Half the population has a MHR higher than that predicted by the formula. While resting HR does decrease with fitness, MHR doesn't really change with fitness.

Just a question - there is a clear increase in HR at the 10 minute mark, then it settles in again. Was this the end of your warmup, and you picked up the pace after that?

Other than that, the shape of that HR graph looks completely normal - a rapid increase in HR in the opening minute or 2, then a very gradual increase in HR over the course of the run.

My guess is that you just naturally have a MHR that is faster than the average (although at 216, still well within the range of what could be considered "normal"). If it was "cadence lock", I'd expect to see a different shaped graph.

Edit: I'm not a cardiologist, or an exercise physiologist, it just looks like a normal shape from my own experince and seeing others post their HR graphs.

There was no change in place/elevation to drive the rapid increase. It’s also odd how smooth the graph gets in the high range compared to the time in the 170s. Every once in a while when my watch takes an accurate reading IMO, the graph is less smooth and stays down in the 170s like the first 10 minutes. Comparing the shape of my HR outputs to friends with Garmin watches, when my HR jumps to the 200s my graph gets too smooth for it to be realistic.

I know this is probably not helpful (since you might not own one), but can you try to pair a chest strap HR for comparison?

I guess things can go wrong with the optical HR on the watch, such as fit on your wrist, different moisture levels etc. Not saying you are doing anything wrong, I just mean that if you've got (arguably) more reliable data you might have a case for warranty replacement with Garmin?

Agree, if this wasn't a race or some sort of all out run... even someone with a very high maximum HR would likely not achieve that type of HR for that long in a training run (unless very out of shape!)   FYI - how in shape you are has no bearing on what your bodies maximum HR is, only your resting HR and lactate threshold HR are metrics that can be trained to change.  

Likely you are just having issues with optical (since you don't mention using a strap), many factors effect optical success which you can read up on various reviews (DC Rainmaker, etc)  darker skin, cold skin, loose watch all make for worse readings.  I always recommend using a strap if you plan to use HR for any type of tracking/training.

Like mcalista said, it does look kind of convincing as an 'accurate' reading, but likely when it steps up to 200+ that is error, was accurate before that maybe.  hard to say, especially as you don't mention how intense or easy the run was.   With 23mpw and not knowing your running history, 7:30 pace might a pretty hard Threshold/tempo pace... so 150bpm wouldn't be accurate likely either.

Read this and any attached links: https://support.garmin.com/en-US/?faq=xQwjQjzUew4BF1GYcusE59 Do a search in the forums for Optical Heart Rate and read some of the posts there too.

Then by a heart rate strap if your want reliable results during activities.

I have the same issue as you show in your graph. I have tried Garmin HR Strap, Polar HR strap and two different optical watches. I have the same on a steady run today, yesterday I did intervals and it was fine. Doctor has not been able to give me any pointers. When I looked back on Garmin I can find evidence in 2016 and I haven't kicked the bucket yet or slowed down. What is most annoying is it messes with my VO2 max. 

You graph is normal in the fact the profile is as expected, but the data is incorrect due to the c20 bpm rise. 

Would love to know what the problem is. Will keep searching.

https://youtu.be/SNZzWgTriCo