The debug stick can't be used to select an "attached: true/false" block state for pumpkin or melon stems. I realize that since attached pumpkin and melon stems are technically separate blocks from the non-attached ones, this is more of a suggestion than a bug, but it still feels like an oversight.
Attached stems are a different block ID from upwards facing ones. Not a bug. The debug stick was added to change states only.
The stems got split up in 1.13 to remove a bunch of unused state combinations (attached and not fully grown).