Space character (" ") on stop-word list acts like a separator instead of a character

[YiFanChen99]

The space character (" ") is already on the default stop-word list.
But SELECT ts_lexize('jieba_stem', ' '); => {" "} instead of {}

Even more, when adding apple watch to the list. Here are the results of ts_lexize:
apple watch => {"apple watch"}, apple => {}, watch => {watch}

Seems that the space character breaks the line into the first lexis only.