Normally you get 15 coins for a win, 10 for a tie. Perhaps it was graded as a tie for some strange reason? When you tie, it still says "you're the winner".
No, you only get 10 coins for a win against a bot (I think you get 15 coins only for wins against human opponents).
A tie would explain the rating loss, though I'd expect it to be even more than 30 points then.
I am now confident it was due to increased uncertainty after not playing for several days that carried the loss. Evidently displayed rating only changes when you play a game while true rating changes daily . Thus if you don't play for a few days a dozen or more point loss (30 here) will be incorporated into your next game.
This explanation requires the player to have initial uncertainty of ~1400. For a regular player, uncertainty is < 300. At +1% uncertainty per day, that's an awfully long time sitting inactive, about 5 months without a game.
Initial Uncertainty = 1400:
AI Rating - Before:
6500.00 +/- 1400.00
Displayed as 3700
AI Rating - After:
6513.54 +/- 1390.26
Displayed as 3733
Displayed Change: -33
Python script to produce this output is attached, including rating info for me and Serf Bot. The trueskill Python module is required.