Interesting. My experience with the S8 is the complete opposite. A Tag Info full scan takes a few seconds (3 - 5) but it works every time. Same with the Vivokey Spark app.
I’m using Spark demo card though, as I haven’t actually implanted my spark yet.
The sweet spot on my S8 is right below the Samsung logo, horizontally centered. I’m still on Android 8. Unlocked, U.S. Samsung phones are always the last to get an update.