Some Garmin watches support 3D Distance and Speed. This feature may cause bugs when enabled with usage of Stryd, and you might not be able to get pace and distance from Stryd. While the 3D Distance and Speed issue is a rare bug, it is recommended to use 2D Distance and Speed if you want reliable pace and distance from Stryd.
If your Garmin watch has this feature, here are the steps to disable 3D Distance and 3D Speed:
Press and hold Menu key
Select activity profile to be customized
Select 3D Speed and/or 3D Distance
Press Start/Stop to Disable