a {
  k: "#{".5"}";
}

is turned into:

a {
  k: "#{"0.5"}";
}

Playground

Expected: The inner string, ".5", should stay unchanged.

This happens because postcss-values-parser does not support interpolations in strings, so it sees the property value as the string "#{" followed by the number .5 followed by the string "}". shellscape/postcss-values-parser#37