Well, I'll guess you must change the way you search.

1. Try six consecutive 1 night non-award stays. If Garden is available for all, then...
2. Try a single six-night award stay for those same dates.

For example, using your dates I tried a six night stay starting 6/7 and it showed Garden available, but trying just the 7th or 8th as a single night Garden isn't available.

...or just call GP desk as this does seem like gaming.