custom syntax hilighting lost on single LaTeX file

Question

I use vimtex plugin for writing tex documents in vim8. I added custom hilight for sections in my .vimrc like so:

autocmd FileType tex :syntax match texSectioning "\\section.*"

The texSectioning is defined in my color scheme file in .vim/colors.

It works on any file with .tex extension except for one! Copying/movig the file to different dir or/and changing name, changing file extention to .txt and then manually switching on tex filetype does not help. Even typing

syntax match texSectioning "\\section.*"

command has no effect. There is no .swp file, I didn't use mkview (there is no view folder in .vim), I didn't use sessions.

I'm really confused by this...

Edit:

the output of set filetype is filetype=tex. The standard syntax highlighting works, I'm only missing he custom one.

Apparently, removing .latexmkrc solved the problem. There must have been some interference between vimtex and latexmk, but it is wierd if that was the case...