What you recorded is exactly what you should expect. The longer you exercise, the hotter you get, the less efficient you get, the more in demand your body is for oxygen and aerobic respiration. It's great you showed your splits because they are perfectly constant! You maintained the same speed but needed more energy the longer you went. Your HR monitor seems like it recorded fine to me (as long as it's a chest one). If you want to double check, record a little extra past your workout to look at your heart rate recovery. If that drops to 120's after a minute it's working fine.

A fun exercise you can try is pick a HR (the lower the harder to keep steady) and try to maintain that. You'll notice you get slower over time ;)

Keep in mind the longer you exercise the easier it is to reach higher HR's. If you sprinted a 1km without warm up and fresh legs, you may not hit the numbers you hit in this work out.

Basically, don't worry too much about HR; just try to transfer zones you feel to your split.