Error detected on BufWritePost Autocommands for "*.vim" : drive not recognized with expand('%:p')

[Ni Va]

Hi,

Using gvim 8.2.292 on windows, following error appears while I am saving a vim file.

It seems to mistake good drive. C: out of  D:

expand('%:p') returns bad drive.

Error detected while processing BufWritePost Autocommands for "*.vim":
E484: Can't open file c:\logiciels\Vim\vimfiles\plugged\foo-bar\autoload\foobar.vim

 autocmd! BufWritePost  *.vim    exe 'source '.expand('%:p')

[Ni Va]

A change dir command has been made before BufWritePost..

How to make expand('%:p') of a file not impacted by a :cd change dir command ?