Shell substitutions not working for substrings

[flipsi]

If I execute :shell echo hello %s on a file filename.ext I will get

hello filename.ext

as expected. However if I execute :shell echo hello %s.foo I will get

hello %s.foo

which is not desired. Instead I would expect

hello filename.ext.foo

---

My use case is is to create a keybinding to create a copy of a file like this:

  { keys = ["y", "b"], commands = ["shell cp -r %s %s.bak"] }

or

  { keys = ["y", "b"], commands = ["shell cp -r %s %s.$(date +%F-%%H%M%S).bak"] }

This may be "fixed" together with the related issue/suggestions here.