expand('%:S') appended to &makeprg after use of make %:S

[Konfekt]

:verb set makeprg says that

vim-dispatch/autoload/dispatch.vim

Line 1329 in a2ff28a

let &l:makeprg = dispatch#escape(request.expanded)

appends the expansion of %:S to &makeprg; instead, keep it as it.

[tpope]

It should be temporarily changed, then restored here:

vim-dispatch/autoload/dispatch.vim

Line 1349 in a2ff28a

let &l:makeprg = makeprg

[Konfekt]

How come line 1349 is not reached?