There's no difference when changing Source to Accessory only or Wrist only. Watch still uses it's native power (wrist source) ignoring connected accessory (Stryd FP in my case). Please make it work correctly. FW 14.13
Ah I see, I have HRM Pro+ so it's taking power from there. Watch is reading it via ant connection, sholud be no problem read it from any ant power accessory then, Stryd included. But Garmin must enable that of course.