Help with Language?

I’m having a serious problem with my keyboard and language setting.

I’m Vietnamese, and since I update to High Sierra, whenever I use Telex or Simple Telex as an input source, this black line always appear under the word that I’m typing. If I’m holding Shift, it prevent me from pressing Space, as I can’t Space while pressing Shift at all. It also automatically disappears if it is the last word of a sentence, which is really annoying. Is there any way to fix this?