Without a clear view of the whole plant without leaves from several other species in the way, I cannot decide whether this is Hypericum japonicum or Hypericum gramineum. The moist site suggests the former.
There are a pair of leaves half way up the stem i the second photo. I looked for leaves further down the stem, but it did not seem to have any at the base. It was growing in a rather boggy area
I am inclined to think it is Hypericum japonicum even though it appears to be upright. PlantNET say Hypericum japonicum has "all leaves hardly stem-clasping, margins flat or undulate". The leaves up near the flower appear to have wavy margins. The PlantNET comment on H. gramineum is "floral leaves tend to be stem clasping, margins ± revolute". This does not appear to be the case.