I discover that args are not "passed through as is by default", as mentioned in the documentation.
By documentation I mean at least what we can read when executing shmake -h.

In fact you need to explicitly ask for it on the command line by specifying -a %s.

This is due to the fact that without being explicit, shmake.exe inject an empty string in the IDS_ARGS resource of shim.exe, and shim.exe pass args only if that resource contains %s somewhere.

I don’t know if we should fix the documentation, or shim.exe or shmake.exe or a bit of the two exe.
And I’m not sure to have more time to invest here since I’ve got a workaround.

But at least it‘s now traced by that issue.