I just spent just shy of 1200 favor points of Mab, then 287 points of favor (I think) with Grendaline to speed up learning Gardening IV. The chunk with Grendaline was enough to max out the speed up. However, then the Rock told me that I had completed the 1511 quest. The total favor I had with Grendaline had been over 1511, then went below that level with the amount spent. However, the total spent wasn't 1511, and was spent with two giants. Seems like a quest bug to me...