I have managed to get "Stroke" to work on my Gear S3.
I had a face, with digital time, and the Hour Right Justified, Minute Center Justified, and the Seconds Left Justified.
Stroke showed on the app, and web interface, but on the watch, only the seconds were showing with the stroke.
The only difference, was the font justification. So, I changed all my fonts to Left justified and the stroke now appears on my watch face.
Hope this helps someone (or the Facer team to fix the glitch)