The following content is extracted from gruvbox.vim. It indicates that green and yellow have been set for different levels of markdown header. However, I can not see the difference from the above picture.
hi! link markdownH1 GruvboxGreenBold hi! link markdownH2 GruvboxGreenBold hi! link markdownH3 GruvboxYellowBold hi! link markdownH4 GruvboxYellowBold hi! link markdownH5 GruvboxYellow hi! link markdownH6 GruvboxYellow
EDIT: Thanks for the comments below! I have confirmed that vim-markdown caused this, what should I do next?