I've been tracking my heart rate while running and have noticed that, every single run, my heart beat has a small, but noticeable, drop between 1:50 and 2:10 minutes into the run (after stretching and a 18minute fast walk warmup). This is on a treadmill at a constant pace, so it isn't me slowing down without noticing. I would have thought it was just a random fluctuation, but again this happens every time. What could cause this?
I would suggest to setup hypotheses and test them systematically.
Do you shift from walking to running very abrupt? Perhaps you should consider it as a spike just before the drop rather than just a drop.
Reason: Your body accommodates to a higher load than needed as a reaction to fast change in load.
How to test: If you walk say at 6km/h then very slow increase the speed to 8km/h. Then start to run at that speed, and slowly increase to the final running speed.
Alternative reason: You feel stressed when you know you have to run in the next 30 minutes.
How to test: Promise yourself to stop running after 30s.
Do you changes running style or step length as a response to increased speed?
Reason: At a certain level you shift to a more anaerobic running style reducing the need for oxygen.
How to test: Make a video of yourself running in the treadmill and compare to heart rate data.
Do you alter your respiration at a certain point? E.g. to make it fit your running rhythm.
How to test: Control your respiration and try to keep it calm the first minutes running.