Why does short-format apply to indefinite articles and how to avoid that?

I would like to use the acro package’s short-format property to print an acronym’s short form using small caps. Unfortunately, when I use the acronym’s iac command, the indefinite article is also included with the format given by short-format. This seems especially unusual since plural endings are not affected by short-format (just as I would expect).

How can I get the short form, but not its indefinite article, to appear as specified by short-format?

documentclass{article}

usepackage{acro}
acsetup{use-id-as-short}

DeclareAcronym{LTS}{
  short-indefinite = an,
  short-format = scshapeMakeLowercase,
  long = labeled transition system,
}

begin{document}
Iac{LTS} is useful.  Here is iac{LTS} to use.
Acp{LTS} are interesting.
end{document}