Your iGS10 / iGS50E is supposed to succeed in locating GPS signal,if it shows speed when you stay still.
But in your case, the GPS signals should be not steady and mostly come from reflection, diffraction, transmission.
It’s nature of GPS that it will show speed in static situation.
To learn GPS features ,see article GPS Drift and Environmental Factors Impact on GPS Accuracy
To prevent showing speed,you may choose the tips below .
1. Pause the recording .
It will no record data when your device pause recording.
2. Stay at places with an open view.
The speed will be accurate when GPS signals are well located at places with an open view of sky.
3. Equip a speed sensors when riding indoors.
More details, see article Why should I equip a a speed sensor?