[Bug] Rainbow have no effect when using in augroup

[M-seas]

Describe the bug

Test on MS-Windows 64-bit GUI Vim 9.0 version
rainbow plug worked well except when using in augroup
the ({[ will become grap or have no rainbow effect

To Reproduce

This is my setting:
au VimEnter,BufReadPost * :RainbowToggleOn
let g:rainbow_active = 1

Expected behavior

Screenshots

Here's the screenshots, ( out of the augroup behavior well but in augroup have no effect

[luochen1990]

filetype？

[M-seas]

filetype？

It's _vimrc file