[Furface]

There's a;lso the possibility of a tuning phenomenon (we don't talk of "problems" do we). What happens to your arrows at the shorter distances? Do they just spray all over the shop, or do they seem to be going right or left? If the latter, it could be your button, and your sight is set for the longer distance.
Also a tad of pussychology. Do you relax for the shorter distance, when there is less "challenge"? I seem to remember some of your best shooting (grouping) to date occurring in heavy driving rain, quite a challenge. This could mean that you revert to bad habits, such as a dead release.
Another possibility. Were you aware how well you were scoring, and anticipating getting a 1st class score? Nothing screws up good shooting better than knowing "If I can only...."
Oh, and well done, from one who has never done it!