Handicap and (possibly incorrect) Stableford scoring

I can see there are a couple of old posts about this, but no clear solution/answer

I've uploaded a few scorecards now since having the S60 where my Stableford score is less than what is recorded (correctly) on the card. I have my local handicap on the watch set to 30, and it's also showing 30.0 in the index/slope, although I'm not sure what that means in practice.

As I understand it (being relatively new to golf), a handicap of 30 means I get 2 shots on holes with a S/I of 1-12, and 1 shot for holes with S/I 13-18

An example today on a par 5 with a S/I of 11. I took 7 shots, so have a nett of 5 which should give me 2 points, but the Garmin Connect/Golf scorecard is giving me 1. Then on a par 3 with a S/I of 16 (I should get 1 shot) I recorded a 5, nett 4 (1 over) and it gives me 2 points.

Have I recorded my handicap incorrectly in the watch or is something else wrong, or do I just not understand how this is working?

Thanks