It is hard to tell without knowing your physicial condition/training status but not totally unrealistic for a human being. Seems a bit high, depending on what you did as exercise...

I'm pretty sure I never hit more than 175 bpm as maximum pulse in 10+ years of sports with HRM belts, but anyway, as suggested in False hrm results, I'd try to check the following points

• Does it have good contact to your chest in all phases of your breathing, a good indicator is when it neither limits your chest movement but also doesn't require adjustment all the time - the latter is probably more an issue with running than cycling, though. Body hair might also impair readings...
• Try to clean the strap with soap and water
• Most of these sensors need some lubrication on the contact patch, usually sweat will do the job, I just lick it before fitting the strap
• Try replacing the battery, some old Garmins gave me weird readings when batteries were dying
• Are you riding with open jersey? I once had an old sensor that showed weird readings when I was going fast (mostly on descents) with open jersey - turned out that the readings went nuts due to wind exposure and the sweat layer vanishing - I only had that with a 2010s Garmin sensor, newer ones didn't show that behavior.

It is hard to tell without knowing your physical condition/training status (and the actual workout done) but not totally unrealistic for a human being. Seems a bit high, depending on what you did as exercise...

I'm pretty sure I never hit more than 175 bpm as maximum HR in 10+ years of sports with HRM belts. That doesn't apply to any other person since heart rates are an individual thing, but anyway, as suggested in False hrm results, I'd try to check the following points:

• Does it have good contact to your chest in all phases of your breathing, a good indicator is when it neither limits your chest movement but also doesn't require adjustment all the time - the latter is probably more an issue with running than cycling, though. Body hair might also impair readings...
• Try to clean the strap with soap and water
• Most of these sensors need some lubrication on the contact patch, usually sweat will do the job, I just lick it before fitting the strap
• Try replacing the battery, some old Garmins gave me weird readings when batteries were dying
• Are you riding with open jersey? I once had an old sensor that showed weird readings when I was going fast (mostly on descents) with open jersey - turned out that the readings went nuts due to wind exposure and the sweat layer vanishing - I only had that with a 2010s Garmin sensor, newer ones didn't show that behavior.

If the readings turn out to be consistent, I would suggest to verify against a secondary source and see if it matches up - I'm not saying it has to be a condition and you have to take a ECG under load but if you have no experience with your heart rate and just started (structured) training, there is no harm in consulting your doctor, there's no harm in knowing how your body behaves and knowing that you actually are a "high-pulser". In structured training, you should know and trust your zones.

Also, as Weiwen mentioned: We don't know what the background of above's workout was, but from experience with training apps like Zwift, you hardly ever go into a hard effort without warm-up to avoid injury. Assuming your true max. heart rate is 210-215, you went into a 15+ minute zone 3 effort with a little zone 4-5 in the end. When you do this on a typical short Zwift workout, there's at least 5 minute warmup before you go through any hard blocks.