I tracked the same route at the same time with my Garmin Edge and the Fitbit App on my phone (Samsung J6; old I know) I was actually checking my phone but it threw up big errors on my Edge 520; it makes me wonder if it is faulty?
You can see the erratic points on this picture (I uploaded to Garmin Connect and took the screen shot from there)
This is the tracking from the Fitbit App on my phone again up loaded to Garmin Connect ( the two variations you can see are correct as I walked on the other side of the road on the return)