Here is my interpretation:
First, miles earned on outbound flight in December do not come into this, as the EQM counter went back to zero on January 1 and OP did not have any EQM's count toward stickers until AFTER the June 28 flight.
Jan 2: earned 19K miles
June 28: earned 18K miles. Total is now 37K and turned GLD, so stickers counter starts on next flight at 12,500
July 8: earned 18K more miles. OP became PLT, but this is not relevant since it does not affect stickers earning. This was the first flight where miles counted toward earning stickers. So, 4 stickers awarded for 12.5K miles and the other 5.5K miles go towards earning the next set of stickers. OP's account should show about 7K miles needed to earn the second set of stickers.