Using any of the Horizontal Line options from the Insert menu (Centered Line, Page Spanning Line, or Signature Line) turns on Underline formatting for the following text.
This bug doesn’t cause existing text to become underlined. Rather, it turns on Underline format even if it was off, which causes typed text after the inserted line to be underlined unless you manually turn off the Underline format.
I am currently using Beta 184.108.40.206 64-bit on Windows 10 Home. I have noticed this bug in the past couple of versions as well but forgot to record it here.